Too add: BitTorrent is heavily "multi-user" in the sense that you communicate with numerous peers (200+ is not uncommon). Each peer gets turned into a small set of processes inside etorrent (4 per peer), so it quite heavily makes use of the process-parts of Erlang. I wouldn't say you need to have to be a "server" in the usual sense to argue the point. In the case of BitTorrent the client is also a server however.