Facebook. A Social networking site available for free where you can interact with people such as your friends, family, co-workers, and just about anybody you know or would like to know.
How? By registering, you can search for and find anybody who is a member (which seems to be anyone with internet access), and add them to your friends list. You can post information about yourself, as well as upload pictures and videos of you and your friends, or anything else you'd like to share.
You can even control access to your information, pictures, and videos, so only your friends can see them.
You can join groups of people with similar interests and share your thoughts and pictures, as well as add many different small applications to add as much or as little functionalty that you desire.
Have a friend, or long lost relative that you haven't spoken to in years? Search for them on facebook, and you'll be surprised! Many people find their lost acquaintances on facebook.
It's so popular that, if you like to socialize, you may find yourself frequently glued to the computer. Everything you need in a social network is available on facebook, from inviting people to partys, to showing off your latest vacation pics, to just plain chatting it up with friends.