foursquare's competitor is other mobile 'time wasters'. I mean that in a good way, not bad way. so facebook, twitter, instagram, etc are all competitors.

if you want to talk about check-ins.. check-ins are commodity, i'm sure foursquare knows this. noone says "hmm.. what app to check in??? hmm what competitors to try?" they go "hmm.. what's interesting and fun way to show off and impress others while i'm here in this restaurant? facebook? foursquare? foodspotting?"