Activity in many forums has drastically declined recently, but I don't think it is because of social networking.
This topic has been discussed quite a lot in many forums.
One factor has been the change in Google's algorithm; they seem to have really downgraded the rankings of forums so are simply sending much less traffic to forums.
I don't think social networks have replaced the media by which people engage in question-and-answer sessions. When people are seeking help, they still need a way to do it. And the groups on Facebook are pretty lame, so I doubt they have replaced mainstream forums.
I suspect there were simply too many forums in the first place. It was another thing for the admins to tinker with and learn about but, after the novelty wore off, they realized forums were more trouble than they were worth. Five years ago, I had several forums bookmarked. Now half of them have been replaced by blogs. The only interaction between users now is via the comments, which is a poor substitute for a proper forum.
Plus, it is simply getting harder to earn money online. People are using more ad-blockers and becoming "ad blind". Because the income a site can generate is shrinking, some forum admins are faced with the hard reality of paying for an ongoing hobby out of their own pockets. I am feeling the pinch myself. I had hoped my site would generate enough income to at least pay for its hosting, but that is a fading dream; traffic is going up but income is going down. I do not know what other way a site can generate income except for ads (paid memberships?). And if this avenue closes, I don't know how long I can fund a site out of my own pocket. I suspect many forums owners found themselves in a similar situation and decided to pull the plug.