And they just basically exhaust all their "favours" from one friend and then ditch them for another?
I have a colleague who does this; she has never particularly tried to befriend me as she probably knows that I wouldn't do all her dirty work for her, but it is interesting to watch her work her way through other employees, usually pouncing when on them when they're new. Said new friend will then be her servant for a few weeks, making coffee for her, going out and getting her lunch, and being at her beck and call, and then if they don't do as she says she dumps them and makes a new friend.
I also have come across a mum from DS's year at school who also moves from friend to friend, wanting constant favours such as taking her child to parties, collecting her child from school etc, and again dumps them when they can no longer meet her needs.
I get that these people are just users, but I can't understand how they can just move from friend to friend like that. Surely they have no proper friendships in their lives if every friendship is based on them using them?