Pardon the vulgarity:
What is really of meaning to you? Your twitters? Your weblog entries? Your list of bookmarks? Your photos? What? Because if you’re not asking what stuff means anything to you, then you’re a sucker, ready to throw your stuff down at the nearest gaping hole that proclaims it is a free service (or ad-supported service), quietly flinging you past an End User License Agreement that indicates that, at the end of the day, you might as well as dragged all this stuff to the trash. If it goes, it’s gone.
I started blogging because I felt I had something to say, and I felt what I had to say was worth keeping control over. My original blogs were hosted, sorta-kinda, on Blogger, but it didn't take long to realize being dependent on a third-party service was a bad, bad idea. If they dried up and blew away, I had no guarantee that my years of blogging would dry up and blow away right with it.
I found a Web host, set up Movable Type, and got to work. I was more than happy to pay something like $4.50 a month for Web hosting, because that made the relationship between me and the Web host entirely explicit. I was a customer. They were taking my money and providing me with a service, and they had agreements in place to explicitly describe what our relationship would be in that regard. My posts were mine; my blogs were mine. They still are.
These days, most people are more than happy to settle for a service -- be it Facebook, Twitter, Tumblr, or the hosted version of WordPress -- as a way to say something to the world. With few exceptions, these services are not designed to allow you any freedom of movement. You can't dump your Facebook posts and take them somewhere; if that was possible, Facebook wouldn't be able to monetize your behavior for the sake of the third parties they sell ad space to. [Addendum: it is in fact possible to dump your posts. But again, Facebook's main motive for hosting your content is to monetize your behavior. You're not their customer; you're their product.]
Most people, I guess, are not this attached to what they produce for the Web. They think of it as ephemera. They have little personal attachment to this or that post, this or that photo -- up until the moment they do, anyway.
If you care about what you have to say, as Jason so vociferously put it, then you have to care about it enough not to entrust your only viable copy of it to people who don't really care about you. I distrust Facebook and Twitter not because I think they are run by evil people, but because the ways they are organized can't help but victimize me. I don't actually own anything I put into those services, and so I never put anything in there I don't want to keep. Linking from those services to stuff I've posted on my blog is fine, because there's a dozen ways to do that. If in five years an entirely different crop of services exists to support such an action, it won't change anything on my end.
Where are Facebook and Twitter -- and Tumblr, and Instagram, and Snapchat, and Pinterest, and all the rest of them -- going to be in five years? Nobody knows, not even the people running those places. An ad-supported model of the Web makes it impossible for them to know, because ad money is fickle as hell. I don't even have the option of paying them to be trustable. They don't want my money; they want me. My present and future behavior is far more valuable to them (in theory, anyway) than any amount of money I could realistically give them.
Trying to build anything I care about on top of those platforms is madness. The only combination that makes sense to me is open source software running on top of a for-pay service. The for-pay service makes explicit the relationship between you and the people hosting your stuff; the open source software makes it possible for you to do anything you want with the data. If you don't like the TOS for one host, you can ditch it and move somewhere else, and the work involved is not terribly complicated.
I'm building MeTal so people who do care about such things have one more option to choose from -- one based on highly permissive licensing, and a different set of technical assumptions than WordPress -- and don't want to feel like it's a choice between Facebook, WordPress, or nothing. Me being one of those people, I ought to know.