My suggestion is to sign up for some of all of the ones I list, and put in just what info you want available potentially to everyone. Check on them all at least 1 time every 6 months, that way if anyone is looking for you... You will be found eventually...
Facebook.com Rating 7 out of 10
Great for finding old friends, relatives, etc. Geared to keep you in touch with people you know well, or people you barely know, or possibly even a friend of a friend. Avoids trying to connect you with complete strangers that are not connected to you in any way.
Lots of apps, pluggins and games.
Praises - Great for sharing vacation pictures / pictures of the kids etc. Great for finding long lost friends. 1 out of 5 people on the web is on Facebook now! (according to Wired Magazine)
Complaints - Interface is confusing, esspecially after recent updates. WAY too much annoying plugin nagging invites to stupid games. Facebook has had a large amount of money donated to them by Microsoft - and forming a partnership. They supposidly sell a lot of your data (Not like credit card info, more statistical about where you are from, who you communicate with, and what advertisements you click on...) This officially makes them part of the evil empire.
According to wired magazine... Many google employees are jumping ship and going over to facebook, hoping facebook will become the new central source for info on the web. I say good luck facebook. :P Google will crush you, and should ;)
MySpace.com Rating 5 out of 10
It's been around forever. Originally geared for small bands to share their music online. Most profiles are completely public. Good for sharing public media files like videos and music that you OWN the rights to and want to get out to ANYONE.
Complaints: This is well suited for extremely bored people who want to be flooded with bot advertisements (usually pornographic)... The openness of the profiles makes your info available to a lot of people by default... So be careful about what you put on here.
Twitter.com Rating 7 out of 10
Twitter is designed primarily for people who want to shoot out comments or updates from their phones to their friends. Though it can be used to follow news sources or updates from a band or celebrity.
Complaints: It took me 5 minutes to get my first 2 porn advertisements once I signed up... So I would suggest you might want to make your profile private, and only allow messages from people you add.
Linkedin.com Rating 8 out of 10
Social networking site primarily geared towards business or school. Designed for finding old schoolmates, co-workers, and friends. Also has a neat feature that allows you to search for someone who works in a particular field and ask them questions. If you want you can set your profile up so that you are available to be contacted by people seeking advice in your field, or for collaboration etc. There is a feature for posting your resume and getting endorsements from other users. (Like a reference list attached to resumes.) Use this site if you are looking for business networking or don't want to put up with all the stupid games and crap on facebook and myspace.
MeeMix.com Rating 7 out of 10
An Online Radio site that customizes the music mix for you. You give it some info about bands you like, it makes up a playlist. As you rate songs it dials in exactly what you want. This is a good way to discover new groups that you will like, while you enjoy listening to online radio. :)
There is a social networking aspect to the site that allows you to chat with other people and view their playlists or stations they have created.
Complaints: Music will stop if you don't rate anything after playing several songs (1/2 hour?) So not great to play in the background if you don't want to be at the computer... Also you may get approached by random people with the networking aspect of this.
Barnes and Noble.com Rating 7 out of 10
You might not think of this as a social networking site... But it is! Sign up for their web page, create a profile, and you can share what books, movies, CDs, etc you own or are reading. Rate stuff you've checked out and write reviews. View other's reviews to help you decide what to check out next. Suggest books or movies etc to friends. It's pretty slick! :)
Complaints: None so far. It just seems a little strange for there to be this much social networking built into a shopping website... But Hey I guess that's not a complaint! :)
MyLife.com Rating 4 out of 10
This is a simple social networking site. I am not sure why you would join this one... It is like facebook but is missing a lot of features. It is simple and less annoying, but less people are on it. So why waste your time? This is basically a people search / classmate finder, and not much more.
Classmates.com Rating 3 out of 10
Why use this when more of your classmates are on facebook? This site naggs you to sign up and to pay to use it. They get your info and then taunt you with a lie about someone who is looking for you and wants to communicate with you if you would just pay for the site to see who it is... I can not support a site who gets the majority of their users through dishonesty and cheap sales pressure tactics. Esspecially when there are better sites available for free. Instead... Sign up for facebook, and add the classmates application. (Not sure of the name...)
There are many others... Some of which are huge. See this Wikipedia Article for more popular social networking sites and specs: