I've used it to communicate with companies and celebrities regarding matters that are very public, and usually ones that require a response from them that can be useful to others or even answered by others. For one, it was about the the end time/length of an event, and for the other it was about the company's services going down. I'd rather that kind of communication happen on Twitter, where perhaps someone will see your tag and answer it before the person you asked, instead of sending an email, filling out a form, or even usually calling a phone number. It's kind of like a terse forum. I also use it on rare occasions to try to promote "viral" activity, like when Stephen Colbert said that The Red Cross (I think it was) would donate money per tweet of a certain hashtag.
In terms of being a viable company I do have my doubts. 140 characters doesn't leave much room for advertising, and extra texts could appear excessively obnoxious, and they clearly need to step up from where they are now since they aren't, nor have they ever been, profitable. Maybe they'll come up with some creative solution.
As for Myspace dying and Facebook taking over... Myspace was designed to be used by musicians and got taken over incidentally. Facebook attempts to do what it actually does, rather than something unintential, and does quite a decent job most of the time. G+ didn't catch on because it wasn't innovative enough for people to bother with it, and Facebook tries hard to move forward and try new things so I wouldn't be surprised if it stuck around for a while. Then again, maybe someone will make something markedly superior tomorrow, which wouldn't necessarily be too surprising. But it would at least be impressive, right?
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