There are some open source alternatives to create your own social networks like Orkut ,Fropper or DesiMartini etc.Most of them are developed with LAMP stack..so you can install and host them easily.

Elgg - Primarily developed for education communities,this open source package allows you to create communities,blogs and share files. You can download elgg from Sourceforge .I had some trouble installing it but once installed ,it worked great.

People Aggregator - People Aggregator is another interesting project.It is available as a hosted service where you can create your own social network.You just have to register to create your community and invite friends or colleagues.But,if you want people aggregator on your own server ,it is also available as an open source package so you can download and customize to your specific needs.

Open PNE One of the finest open source social networking project.But the problem is that its a japanese project with hardly any translation in english.If you know anybody who understands japanese,great project to start your social network.You can download Open PNE here.

Joomla Community Builder I should not call this a social networking project.But this Joomla extension is moving in that direction.A steep learning curve even if you are comfortable with joomla but the result is pretty good.You can download Community builder here.

PHPizabi Another Open source tool with more than 90,000 downloads.Early stage project used for creating social networking,dating and match making sites.You can download PHPizabi here

Update : Found one more project on Digg. Around me is an upcoming Open Source MySpace clone.

With open source options available for developers,I see lot of desimartinis,desichampagne,desibeers and desinimbupaani in future.The runner up(Orkut is already the winner) will be the one who will advertise its website on traditional media like television.