Architecture : How would you go about building an activity feed like Facebook?

I always wanted to ask this question. Now that I am going to work on a project involving activity feeds I would like to ask this one.

• What architecture, tools and strategy will you use to design an activity feed like Facebook?
• There will be multiple users who will follow several topics and posts from these areas should show up in the feeds.
• It should be fast and should probably be cached. Then again what should be an efficient caching strategy here?

Thanks a lot! Any help is highly appreciated.