Speaking from my own experience, I used to post reviews, but I haven't done that for a while. It's useful to keep track of books I've read and books I want to read, but I don't really use it socially anymore other than to reply to messages or comments on my blog posts.
As an author, it would behoove you to snag your author name and set up a profile.
You don't have to have a separate Goodreads blog. If you have a blog of your own on your website or through Wordpress or Blogger or whatever, Goodreads can automatically import your posts and alert your fans/followers by email when you have a new one.
You can limit your giveaways (as long as you have a physical book to give away, no ebooks allowed) to certain countries, which is useful if you don't want to worry about outrageous shipping costs. It's a good way to get your book added to "to read" shelves, but "to read" shelves doesn't necessarily equate "to buy" (and sometimes becomes "that place where I put stuff I intend to get to someday but usually forget about as soon as I click away").
Another bright spot is if you have fans who follow you on GR but not your Twitter, FB, newsletter, whatever, GR will alert them when you have a new book out (they do a monthly newsletter of "new books by authors you've rated").
Advertising on it got me bupkiss, so I don't recommend paying for ads.
Hope that helps.