MongoDb: Using as a graph database for finding "friends" of "friends" for example?

MongoDb: Using as a graph database for finding "friends" of "friends" for
example?

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 scenerio ?
I 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 can mongodb suffice ?
How easy would it be to implement or do I really need to focus on REAL
graph db's.?
I do notice foursquare are using mongodb so I presume it supports there
infrastructure.
But how easy would it be to find all friends of my friends that also have
friends in commun - for example
Any help really appreciated here.