Which is the best social networking software that is cheap and support unlimited users?

[Peter]

I have an idea and i need to establish a social network group/community to engage my users on a certain topic. The Idea i have may attract a huge user base closer to that of Facebook. I need a social networking software that will enable me to scale up my site to enable users to fully interact and be able to use the most advance features like messaging, video sharing documents sharing etc.
Can anyone help me find the best software similar to those used by Ning, Facebook etc

[peter manuels]

I personally like Chameleon http://chameleon.startsocialnetwork.com
It is a rather universal script and not expensive.