I checked out a couple communities with my interests and there seemed to be almost no actual threaded conversations - just users either introducing themselves or saying what they were doing...with, at most, one response.
I think a lot of the secret of "our" forums...that is, the type which have proven successful, is the idea of some sort of clear hierarchy and management. This sets a vision and direction for the community.
Don't get me wrong- there is a place for all this stuff...redit, fb, +, twitter, etc. but I don't see it being where I would go to get information on the wiring diagram for my 1966 Corvette (which I don't own, just an example)...
Our forum, and many others, are niche market segments. I expect slow continual growth of this type of forum.
On the other hand, if someone asks me whether they should start another political discussion forum or game forum, I might tell them it's going to be a tough slog.