You mean use a third party application to manage your social network, not build it in modx ?
There are a few open source engine for social networks, first among them being
Elgg, Barnraiser also builds
social opensource software but most is still alpha/beta,
Noserub is an interresting concept (and it’s built on top of the CodeIgniter framework) but be prepared to dig into code to install
I’d say Elgg is the safest bet, it’s proven and from 1.x and on, really nice except (as usual with social software) templating is not great, code and presentation is mingled
If you need to build a simple social network, I’d advise to do it with modx + appropriate snippets (such as WebloginPE), this will help keep templating flexible and clean