Any social networking site can be dangerous. The best way to deal with this is to create an account of your own, and make your child add you to the friend list with full access. This way you'll be able to see what they're saying, and decide whether or not it could be dangerous, or inappropriate.
A great way to explore the city with your friends and family!
Let me tell you this: foursquare is an amazing website - when put to use correctly.
foursquare is a new generation of social networks that allow you to 'check in' (broadcast your location online) to your favourite places, be it the public park, or a local Starbucks. It also lets you leave online 'tips' for the places. These are essentially mini-reviews or, as the name suggests, tips (e.g. "Ask for the secret sauce!" at a burger restaurant).
Whilst it does have some significant security vulnerabilities - mainly the fact that it openly broadcasts your location publicly - foursquare, if used correctly, can become a great way to explore your town or city with your family or friends.
The bottom line, however, is that, unless your child is about 15/16 years old, do not let your child create your own account. My account is my parents as well, and they control it. It could give away your location to pedophiles, etc. although while the government promotes it, most of the internet is not full of rapists that will abduct your children. It's a very stereotypical view.
The pure reason I have rated it on for 11+ is because it is a brilliant way to find potential new restaurants/places of leisure for you and your family!
Make it easier for pedophiles to know where you hang out!
And they get 5 points for adding their house. 5 points is 5 points, ya know.
Then, some people like to use the shout-outs to say things that aren't always appropriate.
Yes, you can limit your information to friends, but then you take some of the intrigue out of the game. So why bother in the first place?