The reason is that on the Web at large, it's hard to find people. Drobe has an active community of, what, a few dozen members (at a guess from reading the discussion boards), drawn here by a common interest in Acorn & RISC OS kit.
Facebook is achieving a sort of critical mass now; it has so many millions of users that there's a fair chance that you will randomly find mates on there that you already know. I have got back in touch with friends and former colleagues from 15+ yr ago and it's good to catch up.
Myspace is an abomination of a site: badly designed, poor features, and it permits people to make a real dogs' breakfast of their own page.
Facebook, on the other hand, is a model Web 2 site: it's smooth, clean and attractive, plays well with other sites - I suck my Livejournal blog into my Facebook page, and my Twitter microblog, and my Flickr photo library and so on. It also encourages people to use their real names, which is a help.
On the other hand, Facebook's discussion boards are rubbish - no threading or anything. [*Coughs significantly and stares at Chris Williams*]
But it's good for playing online Scrabble! :¬)
I avoid all the pointless "fun" crap - graffiti boards, fishtanks, gardens, zombie and vampire wars, boozemail and so on. I just use it to meet and talk to people.
Which is what communication is all about, isn't it?