This is Vivalogo's list of best free, downloadable, open source social networking software / scripts (kinda hard to say all these words :) ).
i Social is a free social networking script platform that allows you to create your own Friendster and Orkut like sites.
Use bookmarks, dating and create groups with just one mouse click.
Mahara is fully featured electronic portfolio, weblog, resume builder, and social networking system for connecting users and creating online communities. You have seen Facebook, Orkut, Myspace , try Yogurt for Xoops!
No programming required to create your own applications. Include web pages, word processor banners to cash in on your niche.
The Mugshot site lets you track what your friends are doing online across a variety of popular web crm, photos, blog posts, and more.
Web 2.0 bookmarking system, both social (with tags) and private (with folders).
Import and export your bookmarks from Firefox, IE, Mozilla, Netscape.
Create re-brandable, database driven applications mean you can develop professional niche specific apps in a flash!
The People Aggregator is a next-generation social networking system that goes beyond the idea of social networks as mating games, and attempts to use open standards, network inter-connectivity and massive flexibility.
Appleseed is (augmented) social networking software, ie Friendster, only distributed.
Sites running Appleseed will interoperate, and form the 'Appleseed Social Network.' Development is focused on privacy and security, as well as ease of configuration.