And of course I always announce my new books on there.
The problem with the advice to an author to have a blog is that ideally you should have one already, and you should have been posting to it for years. It takes a while to build up a following. Do start a blog, but don't expect an instant flood of people reading it or commenting on it.
General advice: Don't limit your blog posts to your new book or even just your writing career. Talk about lots of different things that interest you -- politics, social topics, movies, etc. My blog tends to get the biggest response, and is linked to the most often, when I publish a negative review of a popular new movie. (Of course, a lot of the responses are rather negative!)
Also, sign up for a free account at Sitemeter or some similar service, so that you can track hits on your blog and see what search terms bring people there.
Also also, follow and comment on other people's blogs. That's one way to get people to your blog. But do it out of genuine interest in what they're saying and a wish to contribute. Otherwise, it looks cynical and manipulative. (This advice also applies to commenting on other people's Facebook pages.)
Also also also, put your blog link in your signature, as I did.
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