Would MongoDB be a good idea for a social network site (developed in Ruby on Rails)?

I like MongoDB and use it a lot, but I am of the opinion that if you are dealing with relational data, you should use the right tool for it. We have relational databases for that. Mongo and Couch are document stores.

Mongo has a serious disadvantage if you are going to be maintaining a lot of inter-document links. Writes are only guaranteed to be atomic for one document. So you could have inconsistent updates for relations if you are not careful with your schema.

The good thing about MongoDB is that it is very good at scaling. You can shard and create replica sets. Foursquare currently uses MongoDB and it has been working pretty well for them. MongoDB also does map-reduce and has decent geospatial integration. The team that develops MongoDB is excellent, and I live in NY where they are based and have met them. You probably are not going to have scaling issues though I would think starting out.

As far as Diaspora switching... I would not want to follow anything they are doing :)

Your comment about graph dbs is interesting though. I would probably not use a graph DB as my primary DB either, but when dealing with relationships, you can do amazing things with them. In fact usually the demo the guys from graph DB companies will give you is extracting relationship knowledge from a social network. However, there is nothing preventing you from playing with these in the future for network analysis.

In conclusion, when you are starting out here, you are not running into the problems of massive scale yet, and are probably limited on time and money. Keep in mind that even Facebook does not use just one technology, they have basically expanded to NoSQL for certain functionality (like Facebook messaging). There is nothing stopping you in the future from using say Mongo and gridFS for handling image uploads or geo-location etc. It is good to grow as your needs change. I think your gut feeling that you have an SQL app here is right, and the benefits gained with MongoDB would not be realized for a while.

My advice would be to use whatever you're most familiar with so that you can get up and running quickly. From your question it sounds like that would be SQL rather than MongoDB.

An interesting possibility for this is Riak. It is a cross between a key-value store and a graph database. The userID could be your key and comments and links could be stored in your value. But Riak also has key-to-key linking. This could be used for connecting up your users as friends. The linking is asymmetric so you would need to deal with adding and deleting in both directions, but that shouldn't be too hard.

But note that Riak is not a document datastore, which means it is value-agnostic, which means it won't help you extract internal parts of your value, which means if you want to pull a comment out you'll need to first retrieve every comment and link that's stored with the user.

You may also want to check out other graph databases. Social graphs are the prototypical use-case for a graph database.