Define "best"? elgg is a Ning alternative that's got pretty wide usage status.net (formerly identi.ca) is an excellent Twitter alternative Appleseed, Diaspor

Define "best"? elgg is a Ning alternative that's got pretty wide usage status.net (formerly identi.ca) is an excellent Twitter alternative Appleseed, Diaspora, and OneSocialWeb are distributed Facebook alternatives crabgrass is targetted at activists depending on what you're looking for, any of them might be the best