harryh:

Twitter and Foursquare offer this too. But I trust Twitter and Foursquare never to give the info to outsiders and certainly in not a sketch way. They’re also private companies offering a free service.

FYI: Foursquare will share your phone number and e-mail address over the API to friends of yours. If you would prefer not to do this it can be turned off from http://foursquare.com/settings. It’s on the very top of the page, in a hopefully very very easy to understand way.

Also, as always, you can read about foursquare’s privacy policies in clear, easy to understand language at http://foursquare.com/privacy.