If you're perusing this blog, you probably love to read. As online communities grow and the websites supporting them become more sophisticated, niche networks are cropping up. There are a number of sites for readers and book lovers.
My personal favorite is Library Thing, which lets you catalog your personal collection (or just what you've read), post reviews and see what other people are reading. There's a Suggester, to suggest things you'll like and an UnSuggester, which offers up what you might not like.
There are a tremendous number of sites like LibraryThing, each with its own following. Shelfari, Booktribes, and Goodreads are all growing in popularity.
For more real-world interaction, there's BookCrossing, which bills itself as a "catch and release for used books". Members read a book, post a review and then leave the book in a public place for the next book-crosser to find. Each book is given an identification number and its journey is tracked through the site. Don't try this with library books, please!