1) Hash tags
Plenty of free book-related Twitter events take place every day, many of which involve sharing an image or a sentence of two from your story. Most forbid buy links in the tweets; however, they offer a chance to share your work and maybe attract some new followers.
Find some suitable (for your genre) book promo websites and bloggers and join their mailing lists. From their emails, you may learn of upcoming opportunities such as author events, blog hops etc.
3) Author/Promo Groups
Many sites such as FB and Yahoo have groups that deal in cross-promo opportunities. This is generally on a quid pro quo basis, where you do something to help promote another author’s work and they will reciprocate. There are also plenty of groups that list upcoming events for authors, submission calls etc.