Many of the applications (in fact all of the ones that I've been invited to use) require you to give access to ALL of your profile information. If you are happy to share that in an uncontrolled way then go ahead; otherwise be very cautious. Similarly Facebook will locate friends for you if you grant access to your various address books and contact lists. Once more you think you might be in control, but how do you know?
Facebook can be fun and a good way to keep in contact, but like all computer activities, be very careful how you share you data. Remember that a loose e-mail could be world-wide in seconds.