And where will facebook be in even 10 years? Take a look at myspace now and that'll give you a clue.
You really need to understand why Myspace went out of business. It was aimed at kids, had an awful layout, and a layout that with very little input could be made even worse. But what killed it was Facebook.
Facebook was aimed at adults from the start and had a very limited way to customise it's layout, which made it readable.
Absolutely wrong. Facebook was aimed at college kids... made by college kids... and for years you had to have a .edu email address to register!
Myspace's downfall really is not a good example to project onto Facebook. Even Google+ has been unable to shift Facebook.
Why will facebook face? Kids are leaving it and kids aren't joining it as they get older. They've lost 11,000,000 teens in the past 3 years which is staggering because that means new kids aren't joining and kids that were on it deactivated their account! They left primarily because their parents, adult relatives were on it. They've since gone to twitter. Because few kids are on it, it won't last into the future if there's only 30+ people on it. Even adults will move onto something new.
2 years ago many news organizations and news personalities listed their facebook page. Instead they list their twitter account instead.