I agree with the logical portrayal of forums as libraries and FB etc. as daily papers.
Theoretically, you go to a library to look for whatever you need to know, but, according to my experience with the forum vs social net, people tend to ask same questions again and again in the daily newspaper, rather than go to the library i.e. forum. So there is no need to scroll down to find a story that was published six months ago - you just repeat your question and get a new answer, often from new people and sometimes a better answer than six months ago.
I've lost one huge discussion board to a social network group that I myself set up to promote the board in the first place, and now amstruggling to keep going a smaller version of the same board, by adding lots of off-topic forums to make it more attractive. It does attract lots of spammers, but Facebook users do not return.
In my opinion, people have changed along with the social networks. All they want is to post their stuff, get a couple of likes and continue posting.
Besides, even with niche subjects, you do not need to go a forum with your question, the Internet now has all the answerss, just Google.