I created an index on cluster, cluster_depth, visited (ascending on all), and querying I get this:

In [60]: people.find_one({"cluster": cluster, "cluster_depth": 0, "visited": {"$exists": True}})
Out[60]:

{u'_id': u'Boytrash', u'cluster': 0, u'cluster_depth': 0, u'friends': [u'b1u1rner', u'javierodrigo', u'Xpato__X', u'flugu'], u'got_friends': True, u'real_name': u'Roberto'}

Visited does not exist, it is wrong. Removing the index gives me the correct reply.