It will someday go the way of MySpace though. Might take a little longer due to mobile addiction, but it will.
Keep in mind, the reason MySpace "went out", as the saying goes, is because MySpace was ultimately a terrible service. The code was dated and slow. Users could customize their pages to the extent that they typically bogged it down with videos and music embeds that slowed it even more. (And was never valid embed code.) And the ability to use custom CSS resulted in some of the most god-awful color combinations you could ever possibly imagine in your worst, fever-induced nightmares.
Add to all that, there was nothing to do there. There was no feed to see what your Friends were doing, and they didn't implement one until years after Facebook made it the standard.
Facebook isn't killing forums. They're not going to Facebook for discussion of their favorite niche. They're going for a one stop source of all their niches, friends and family's up-to-date info. If they have an interest in a particular topic, they'll go to those sites. After all, the internet is a big place. If being able to do all the stuff you do on Facebook killed all the other sites that specialize in what Facebook also does, Facebook would be killing the internet. No, people will seek out their favorite topic in spite of Facebook, and they'll visit your forum if they 1) find it in the sea of forums that already cover your topic, 2) have an interest in actually discussion that topic 3) are attracted to the vibe of your forum. Which are the things you had for and against you long before Facebook existed.
What Facebook might be killing, though, is off-topic, general discussion forums. Those were tough to build in the first place. I can't see them thriving in a Facebook world, easily.