Foursquare is an interesting and a potentially very dangerous concept that has upsides and severe downsides. The general complaint that has been voiced repeatedly is that it allows villains to know when somebody is not at home. Certainly it is helpful for clients to know where a trader is - particularly if that trader is a sole trader providing services.
What happens if a villain also uses the internet? Villains are now very tech savvy and have smartphones. They can check to see where the people that live in various houses are if their whereabouts are posted online. Thus this gives them carte blanche to perform their evil, secure in the knowledge that their victims are miles away.
Foursquare is a double-edged sword in many ways. In general it depends upon clients actually having Foursquare as well as being able to understand it. Add in the security chasm that Foursquare opens and it becomes much less attractive. Imagine that instead of a face painter whose equipment costs very little, it was a photographer with all the latest equipment. What a haul that would be for some workshy drug-addict!
Facebook has exactly the same kind of checkin facilities as Foursquare. Facebook's security failings are well known and worse than that, most of the criminals use Facebook also. Does this site use Foursquare - certainly - but the checkins don't always reflect the location of the author.