db.users.find({'friends':[]});

This does not work! I would like documents that don't have anybody in "buddies" attribute.

It can be done easy using $size operator:

db.users.find({'friends': { $size: 0 }});

Try

db.users.find({'friends': {$size : 0}});

Read about this here:

http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%24size