I used to feel the way that you do....It seemed almost silly to start new forums for the discussion of different topics....but ultimately, having more forums on my ...ummm....forum....has made it more active. Instead of just logging on and posting a thread or two in the general forums, someone might log in, post about something in the general forums, go to the parenting forum and ask a question, go to the marriage matters forum and discuss something important that just happened and before logging off...add a book review in the library forum. Adding private forums is even better because it allows for more private discussions outside of the public eye.
Segmenting things makes it easier for people to organize conversations.
I went from having a forum with one or two major forums as a part of it to having many...and my experience has truly been "if you build it, they will come".
I only have about 800 users and at any given time, we have ~100 active users....