Recently, with Facebook coming out hard with their Places and more specifically Deals, people are heralding the death of 4sq.
I see two problems with the notion in the short term and two in the long term.
Most people using FB are plopped at a laptop or desktop, not wandering the countryside which is the POINT of LBS (location-based sevices) or geo-locational deals. They are the customers.
The apps and gizmos are REALLY geared for the locations to make use of. Most of your friends don't care if you checked into a restaurant, but that restaurant will- once they realize they have a decent tracking mechanism.
(And yes, I know that its a small percentage of locations/venues that are plugged into the locational stuff, but that is a growing market.)
Foursquare is countering with their own deals etc and as mentioned, it will not be difficult for MORE businesses etc to get on board.
Foursquare and Facebook are essentially two markets of people for the most part. 4sq is clearly a mobile thing, while Facebook is behind the eightball, but with more users. Mobile growth is fairly certain so both need to get into it.
The second reason long term that Facebook isn't killing Foursquare just yet? Someone will come up with an easy way to do both. Even if FB PRECLUDES your 4sq checkins, there are always work- arounds: look at check.in.