I'm genuinely baffled about what the troll gets out of it?
Well, it's the chance to indulge in a mad fantasy life, isn't it? And these people never have mundane lives, they have crazy, drama filled lives and the more dramatic they are, the more attention they get from the unwitting people who believe and support them every day.
I once saw the most unbelievable, complex series of stories and characters come crashing down on OpenDiary (God, anyone remember that?). When I look back on it, it's amazing that no one twigged earlier because it was SO far fetched - two male ex-lovers and their subsequent female partners, having children and all sorts (each new person on the scene ended up opening their own diary on the site soon afterwards, funny that! 
). Anyway, one of the original male lovers "committed suicide" and as they were big players on the site, it was a big news and lots of users were distraught at the loss of a close Internet friend...
Turned out that none of these (maybe half a dozen?) people were real and the whole thing was orchestrated by one woman in, I think, Australia. She'd been at it for YEARS. She'd even set up fake marriage announcements on other sites to give credence to her characters. Madness!