I'd say a graph database would work much better for building a Facebook clone compared to a document store like MongoDB... And a relational database would still be better than the document store.

I've been using Mongo happily to provide online analytics solutions, but the advantage is mostly from the development side, not really from the performance side.

On the other hand, this kind of approach is great for attribute matching, which is usually a nightmare to do properly with a RDBMS.