I have been investigating a graph database and I have found neo4j and although this seems ideal I have also come across Mongodb.

Mongodb is not an official graph database but I wondered if it could be used for my scenario.

I am writing an application where users can have friends and those friends can have friends etc, the typical social part of a social network.

I was wondering in my situation whether Mongodb might suffice. How easy would it be to implement or do I really need to focus on REAL graph databases?

I do notice foursquare are using Mongodb so I presume it supports their infrastructure.

But how easy would it be to find all friends of my friends that also have friends in common, for example?