People have been saying this for the last 5 years.
I think it's time to reiterate a point that was made here a couple days ago that I think really outlines the reason that Facebook WILL be around for a while.
The comment that I'm referring to suggested that there's a "point-of-no-return" for a service where people are too deeply invested in it for any competing service to have a REAL fighting chance. That point of no return is when you have grandmas and grandpas, as well as little kids, and non-tech-savvy folks in general using the service (in this case FB) consistently and constantly.
To give you a personal example, my parents are the LAST people who I would expect to create a Facebook account. They are very technically unsavvy, but on top of that they're pretty skeptical people too. However, they finally got a Facebook account, and I think that highlights why Facebook WILL be around for many more years.
If you're going to use Myspace as an example, then don't, because Myspace NEVER reached the tipping point of Grandmas and Grandpas trying to use it. I think this goes for any service, not just a social network like Facebook. Take the iPhone and iPad for example. Same thing with that. You have grandmas, grandpas, and little children using iOS, so you know it's ingrained across multiple generations.
Now I'm not saying I'm 100% sure they're a rock-solid company, but I don't think people give them enough credit. It's far more popular to say "oh they'll be gone in a few years" than to actually understand and analyze the situation.