hort definition:
The first thing to do is to define a spam detection rule. If a message contains a URL and is shorter than 500 bytes than it's spam. Now we can apply this rule to postings in a website. If less than 5% of the postings are detected as spam, then it's forum. If more than 95% of the postings are detected as spam then it's a social network.
long explanation:
From a technical point of view, a forum and a social network are realized with the same software. The user creates an account, can post something in a group and other users can answer. The difference between phpBB which is a forum software and yammer which is an intranet social network are minimal. I would say, from an abstract level it is the same software. If somebody doesn't likes to install yammer, he can use phpBB for the same purpose and vice versa.
On the other hand there is large difference between a forum and social networks. This difference can't explained on the technology level but has to do of how the technology is used. The guidelines what is allowed and what is forbidden in a forum are usually written at the info page, and if no info page is available the forum itself provides examples. That means, all the forum posts which were not deleted by the admin are following the rules.
To analyze the difference between a forum and a social network we have to describe the guidelines of the forum. I would like to provide an example info page for a forum and for a social network. A forum has usually the following rules:
1. marketing and spam postings gets deleted
2. the user should ask question, and the other user should answer these questions on a knowledge level
3. additional links to blogs and other websites are allowed, but urlspam gets deleted
4. the postings are organized in categories for computervision, machine learning and game AI.
If we are taking a look at real forums in the internet, we will notice that most of them are operating on these rules. Only in minor details the rules are slightly different. The idea behind the rules is to allow a community to discuss topic and prevent that the forum gets overtaken by spam bots. The admin has same tools to monitor the actions in his forum and he is allowed to ban users.
Now the rules for a social network:
1. posting content or questions is not allowed
2. the desired post contains of a URL and an short comment
3. the posting fits into groups
Let us compare the rules against each other. The obvious difference is, that in a forum content is posted, while in a social network the URL is posted. From a cynical perspective a social network is equal to a spam bot forum. The rules of a social network are prefering the behavior of spam bots. To explain the situation let us make an example:
A posting is submitted to a social network which is Google+. The posting is written in a style which fits usually to a forum. That means, the poster explains in 3 paragraph his problem about programming a robot with python. What will happen next? The first important thing is, that this posting violates the rules of a social network. Rule 1 says that posting content or questions is not allowed. The second violation is, that the asker didn't post a URL at all. Instead he has formulated his programming question without redirecting the user to external content. From a very strict perspective the posting should be deleted. In most real social networks like Google+ this won't happen. What will happen is, that the posting gets ignored because the format is not normal. And if the posting gets answered the answer is not a answer but looks similar to a comment and a judgment about the question. Social networks are not able to discuss topics by the topic itself. Social networks are organized in a way to connecting people and to redirect traffic to URLs.
phpBB
In the introduction it was mentioned that forums and social networks can be realized with the same software. What is different is the communication style. Forum posts are containing large amount of text, and no or only additional references. In contrast, social network posts are starting with the URL and containing only a short amount of text.
The difference between a forum and a social network is made by the guidelines posted at the infopage and the admin who makes sure that the user are operating with these guidelines. Each website has to decide between a forum or a social network. It is not possible to become both.
Detecting spam
On a formal perspective the definition between forum vs. social network is made around the term spam. Spam is a certain sort of posting which can be detected by the admin and by automatic software. In a website there is a list of postings, which are numbered from 1 to 100. It is possible to set a spam flag at a posting. If the flag is set, the message can be deleted.
The question is, what is the criteria for the flag? I have mentioned previously that the criteria is different in a forum and a social network. Most admins belief that they can handle spam. This is only partly true. The precondition to detect spam is, that first a definition was made. Let us give a short example.
Suppose the definition of spam is, that the message contains a URL and the total length is smaller than 500 bytes. This rule allows a human or an automatic script to analyze each posting. If this rule is applied for a quality forum, it won't detect any problems. All messages are clean. If the same rule is applied to a google+ group the spam detector would produce an alarm for 90% of the messages.
Spam in social networks
According to the definition from the beginning social networks are containing posts with a URL plus a short text. What is the purpose of such messages? Let us take a look at the URL. An url is an internet technology which is referencing to external websites. The reason why the URL is posted to a social network is because the external websites is part of a distributed community. If the URL won't posted to a social network, the external website is not able to find friends.
Let me give an example. What would happen if a blog community of 100 blogs would remove all the links which are directing to each other? The content in the blogs remains the same. What will happen is, that the traffic between the blogs would be lower. A stranger is reading blog1 and finds some useful information. He has read the entire blog and then the journey stops. No links are available and he doesn't know from the other blogs. He can logout from his computer and something which is more pleasant.
If the inbetween links gets activated, and if the blog community is announcing updates to social network the traffic will grow. The blogs are connected to each other and this improves the visibility. The main reason why URL are posted to social network and the main reason why companies are spending money for ads is because it works. If all the spam-bots and all the backlinks doesn't produce a result, noone would invest time and money into the subject. The question is not if social networks are useful or not, the question is how to optimize the usage.