Twitter (the service, not the company's) secret sauce is:
You can have a public platform, using an identity you choose for yourself, to broadcast snippets of information.
Where do you evolve this? Blogger, Blogspot, Livejournal, Tumblr, Medium, they're all in this same space, except they focus on traditional, long-form content. I don't see Twitter as a social network (although many do, that's okay). I see it as a broadcast medium, a soapbox, a blogging platform.
But if you see Twitter as a social network, you're subject to a different set of pressures than a blogging platform.
Also, Twitter holds no value to FB, Google, MS, or Yahoo. They all have subsumed parts of its core functionality into their offerings. If they ever get acquired, it's by some 'outsider' that wants to enter this space, like Verizon or Yandex.