mongo db or redis for a facebook like site?
These things are subjective and can be looked at in different directions. But if you have already decided to go with a nosql solution and is trying to determine between mongodb and redis I think it is better to go with mongodb as I guess you should be able to save a big number of posts and also mongodb documents are better suited to represent posts. 
Redis can only save upto the max memory limit but is super fast. So if you need to index some kind of things you can save posts in mongodb and then keep the id's of posts in redis to access faster.
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.