A Social Network In Rails: a gem by gem guide

I'm writing an in-depth guide on building a social network with Rails, one gem at a time. Here's what I've got so far, would love to have some feedback!

1. 5-minute Setup with RVM and Railsbricks
2. Photo upload with Refile (coming soon)
3. Testing: Fixtures
4. Elegant permalinks
5. Activity Feed
6. Comments
7. Notifications (coming soon)
8. Follow/mention people (coming soon)
9. ...

I'm compiling and expanding these blog posts into a book via Leanpub, A Social Network in Rails, subscribe to be updated of its progress here.