Joining various MySpace "groups" is perhaps the best way to find new friends. From MySpace.com, click Groups on the top navigation bar. On the left, you'll see a link for Search Groups, where you can search for your genre, topic area, favorite authors, etc. Join groups that reflect the type of books you write or like to read, or other topics that interest you.
Joining groups is a better way to connect with potential readers than just randomly sending friend invitations to any profile that you happen to see. Some groups allow you to post bulletins where you can mention your book. But check on this: It's important to know the group's terms of use, and you don't want to be accused of spamming the group.