BuddyPress.org

Totally agree with Peterverkooijen.

Remember that the freedom and control over you stuff that you get thanks to a self-hosted platform (like BuddyPress) is really priceless.

Before you start your net you could try playing around with it enough to master the basics and more.

If you have used WP, you will have no issues at all installing and running BP.

Stay away from Ning, SocialGo and other external providers, they wanna make money out of it, dont forget it.