How Did Foursquare Get People To Check-In?

Here is a puzzle we never understood: the concept of Foursquare is based on check-ins. It is a great app, can tell you tonnes of things in a very personalized fashion etc, but the whole concept is based on people willingly sharing their location details via 'check-ins'.

That is a classic chicken-egg problem - any value that Foursquare can promise for someone checking in is based on other check-ins (at least the social part). And the complete local search piece ('show me pubs in the neighborhood that my friends have visited but I haven't visited so far') is also based on check-ins. That is Foursquare's currency.

Yes, there were badges and gaming mechanics and mayors and stuff - but seriously, people checked in for that? They gave up their location information for that?

We are curious to know, how did Foursquare get people to do all those check-ins? That would be a goldmine of information for everyone trying to get concepts off the ground, that depend on getting people to 'participate and contribute'.