These games actually do really well, not from banner ads but from virtual currency. Now that FB is pushing devs to use FB credits, they should be in for a healthy cut of the action.
Facebook's core value is that (nearly) everyone is on it and it is considered to be "cool". The cool aspect will not last. I'd be amused to hear any arguments as to how it possibly can. Young people will move on to something else.
I also think it's very likely that their power as a centralized manager of personal contacts will be eroded over time. Really, it's not that hard of a problem and some other applications will end up being compelling enough for people to switch. They will try to prevent it (e.g. like trying to prevent GMail from exporting email addresses) but ultimately will fail.
The history of the Internet is filled with the corpses of technologies that were supposed to be "the future". Who remembers PointCast? How about Orkut? For a while RSS was touted as the solution to all of mankind's problems. ;-)
Well, sure they're fads. But toys aren't a fad.
LOST might be a fad, but entertaining content aren't a fad.
Farmville might be a fad, but Zynga is in the content business, not the Farmville business.