mongo db or redis for a facebook like site?
First, loosely couple your app and your persistence so that you can swap them out at a very granular level. For example, you want to be able to move one service from mongo to redis as your needs evolve. Be able to measure your services and appropriately respond to them individually.
Second, you are unlikely to find one persistence solution that fits every workflow in your application at scale. Don't be afraid to use more than one. Mongo is a good tool for a set of problems, as is Redis, just not necessarily the same problems.
I'm building a social app in django, the architecture of the site will be very similar to facebook
There will be posts, posts will have comments
Both posts and comments will have meta data like date, author, tags, votes
I decided to go with nosql database because of the ease with which we can add new features.
I finalized on mongodb as i can easily store a post and its comments in a single document. I'm having second thoughts now, would REDIS be better than mongo for this kind of app?
Update:
I have decided to go with mongodb, will use redis for user home page and home page if necessary.