I am a member and have several friends on Facebook - but since you have to "know" a person to request friends, how do you really make a network?
Promoting your profile so people who "know" you can request you to be added as friends. This promotion activity is often made by inviting people to visit your profile, as in example, using your forum signature for it.
I have found it very hard to promote effectively through myspace. Best way to promote though is starting a group and trying to get people to it.