Even sorceforge and hotscripts overlap with some OSS. Hotscripts may have more small scripts and sorceforge may have more complete systems. So you should check out sorceforge too.
There are a lot of php OSS script or system in social network or community. You may have to dig out and compare several of them.
To name a few
1) elgg - A framework.
2) dolphin - a CMS with rich features.
3) phpizabi
4) ExpressionEngine - forum, wiki are good. But their license about blog hosting is an issue you should check with them.
Let us know your findings or selections. I am interested in finding which of these system fits which cases too.