Watch for hackers!
Hackers are common on these sites and you have to be alert.  If you get hacked chances are they will spam through your account.  This could cause problems for you.  If you see you are hacked notify administration and make sure that you let all your friends know you were hacked.

Choose a reliable service
Make sure you choose a site that is trusted and proven.  You do not want to go with a site that is questionable in any way.

Understand the network
Know what the atmosphere is like. Understand the type of person who frequents the site and make sure you are a good fit.

Take advantage of groups
Join groups.  It is a great way to reach your target market.  It is also a great way to build up your friends list.

Fill out your profile
Make sure you completely fill out your profile.  You do not want blank space on your page.  Plus filling it in gives people information about you and helps them get to know you.