A agree with RoJoHen... Facebook allows businesses to connect with consumers in ways never before conveniently possible. THAT won't go away anytime soon, unless something else replaces it.
Twitter and Instagram are all highly geared for fleeting communication. It's great for quick updates, but not for building up a cache of information. It's no wonder teens are finding it more appealing, as they seem to cater to things that move fast and don't linger. ;-)
I'm also of the opinion that Facebook has made too great an investment to fizzle out. Some people have overdosed on it and... well, they eventually had to pull back. If you and a bunch of people who know had to do this, then it's easy for you to conclude FB is on the down turn. But it's all about perspective. I think a majority of people have throttled their usage of FB to where it's manageable.
But... what Facebook is all about is making money. They really haven't cached in on their grand plans yet. The trick of it is more ways to make money from advertising. And at some point again they'll be tripping over themselves as they try to put those plans in action because ultimately people don't want unsolicited advertising.