Comment by MehdiEG

Using the Foursquare API for this sort of data aggregation service (i.e. for stalking people) has been done many times before. Unsurprisingly, Foursquare didn't like it one bit [0] and ended up changing their API to prevent this sort of usage [1].

[0] http://aboutfoursquare.com/foursquare-api-change-girls-aroun... [1] https://groups.google.com/forum/?fromgroups#!topic/foursquar...