The two services, while similar, have different levels of reach. Foursquare, which started in only a handful of cities across the US, has a devoted user base, actively seeking badges and mayorships and taking advantage of special advertiser promotions and deals while using the service. Facebook's equivalent doesn't have badges, mayorships, or any kind of rewards program for check-ins (yet).
I'm a devoted Foursquare user but have used Facebook Places (maybe) twice, and I don't see that changing any time soon. What about you? Have you checked in on Facebook Places? Foursquare? Both? Neither?