So I'm considering joining Twitter.
The strength of Twitter, in my opinion, is in "watching" live events.
Example: A couple of years ago, my mom traveled to Hawaii for the first time. She flew out on a Friday. That Saturday morning, there was a tsunami warning. I "watched" the whole thing unfold on Twitter, reading news from people who were there. It was comforting (and luckily the surge was negligible and no one was hurt).
As a marketing platform, I don't know how effective it is. Clearly some people seem to leverage it successfully. I buy hand-made earrings from a woman in Washington State, and she tweets every time she's got a new design. On the other hand, there is one self-pubbed author I follow (who shall remain nameless, because he seems like a decent guy, and I'm about to slam him) who posts almost nothing but links to reviews of his books, announcements of sales, and naggy things like "It's cheaper than a cup of coffee!" He does this multiple times each day. It comes across cheap and desperate, and makes me think his book is the same.*
I don't tweet much, but I follow a lot of people, and I get a lot of news there that I wouldn't see otherwise. I like Twitter, and I'm sure some people can leverage it successfully for self-marketing; but it's hard sometimes to see how.
I think the secret is to tweet about a lot of stuff - not just your work - and get followers who think you are interesting. Then when you drop in that "Oh, by the way, here comes that book I've been working on in between tweeting pithy things to all you wonderful people" link, it's more organic.
*I started his book, but I did not finish it; IMHO he really, really needs an editor. Not so much bad grammar, but sloppy grammar, and right at the start when he needed to be pulling me into the story. Might otherwise be decent, though, so my loss, I'm sure. 
Congrats History!!! 
But I think it was good. 
