In the past communication between communities and the customers was done by phone calls,IRC chats,then emails and forums.Then with the increase of tech savvy crowd in the world more social sites came into existence.Forums have stayed from a long time and are still a good resource of knowledge and communication.
But with the advent of Facebook the big brother of social sites and twitter the favorite and more preferred by a huge mass more and more communities,groups and companies are hosting their own forum sites as well as maintaining their Facebook pages and twitter accounts.I do not by any means oppose the Facebook pages and twitter accounts the companies hold,but this trend its own cons like particular groups and companies being mentioned/tagged in irrelevant/hate speech posts which add to the amount of work needed to get in contact with genuine customers/members.
The official Facebook page/twitter account of community/company/group does get mentioned in search engines but a particular post for which a person has searched the internet may not be visible in search results as per the rules of twitter and Facebook the community’s privacy settings and that of the person who has posts.
Forums are more open whether the thread is a flame war or appraisal for an achievement the page will be listed on search engines unless of course it is deleted trolls i.e fake/abusive accounts can be banned by their user names,email ids used or at last via their IP addresses if same problem is mentioned different posts newer posts get merged/deleted so redundancy does not get much a chance while in a Facebook page or twitter mentions it’s the same thing asked again and again and again by people and it takes the same delay on a Facebook page as it would take in a communication via email.
So I do not get a point in making a Facebook page or a twitter account where people share about a product/service and get their queries solved if you already have invested in a forum site why to invest more in a social page.