Let's face it. Facebook places has not taken off like most thought it would. With 600 millions users, Facebook has failed to make a dent into the geo-location check-in wars. Even Gowalla has had more success than Facebook. The downfall to Facebook Places is that it offers virtually no incentive to the user to participate. Recently Facebook Places collaborated with REI to offer users an incentive to check-in. For every check-in at REI, REI would donate $50 to local charities. Money talks. Otherwise the only incentive to check-in at REI would be to inform your friends where you were.
Foursquare may have started as a game but it has evolved into the place to be for companies. From it's simple yet addicting badge system to it's foursquare specials, foursquare offers nothing but incentive to participate. To put it in simple terms, it's just plain fun. It makes a lackluster day of errands or business calls into a war for mayorships and a hunt for the next badge. This may seem like a silly web 2.0 service but many Fortune 500 companies would beg to differ. foursquare specials are used by merchants to get people into their stores to buy goods. Facebook has yet to make specials in Places simple.
If Facebook purchased foursquare, it would be able to merge the best of both services. It is not Facebook's way to just purchase a company that does something better than them, but in this case they should. I love foursquare and would hope it would stay the same if bought out but I also want to see them succeed. Merging foursquare into Facebook seems like the next logical step.