Here's my opinion on this, having been there and done that.
1. Reading groups in the social networks such as Facebook and MySpace are a great place for an advert, providing one of the following:
(a) You are already a participating member of that group
(b) The group policies encourage promos
Your advert should include, if possible, your jacket blurb, a cover shot, and a very short excerpt (less than a page). If the forum has a separate bulletin area like MySpace, post there instead.
2. Writing groups are not a good place to promo - especially if you are not a regularly contributing member. The only exception is if you are a regular, and the group's policies tolerate such promo.
3. You do have your author's website up, yes? It should contain the cover, blurb, back page blurb, and a complete first chapter. The web page should be part of any electronic signature such as email and forums (if allowed there). Business cards help to.
4. SF conventions/conferences. Get on a panel.
5. Reviews. Good luck there, but if you can get one from a blog, or be a guest on a blog - do it.
What not to do is just as important. Do not spam forums and groups whose policies frown on promo or are not reader forums for what you are selling.
Good luck!
Kerry