timtowle

If you don’t want to build your own, there are alternatives out there, from Open Source programs to hosted pay-as-you-go services

BarnRaiser
BarnRaiser describe their software AroundMe as the ONLY free social networking and group collaboration platform available today!
BarnRaiser not only creates Social Network software, the Barnraiser Foundation actively creates projects to support real life communities.

People Aggregator
Project of Social Net evangelist Marc Canter, People Aggregator enables you to create a Social Network and blogging system – they offer both hosted and downloadable version. The license is curious – you pay them only when you’re making money.

Community Server
Community Server is the software developed by Microsoft, that their famous Channel9 MSDN is based. Spun-off into a separate entity it now offers hosted and software version.

GoingOn
Promises that you can host your network in their new environment, still to launch publicly.

Ning
Ning, a Marc Andressen company, aims to be to Social Software what Typepad has been to blogs. It’s a fantastic product, with a few drawbacks that have stopped it hitting the bigtime – no domain mapping and users have to register with Ning, not your app.

Sparta Social Networks
Creators of the (once) famous Backwash.com, a fully fledged Social Network white label company. Accessible and flexible price plans for the hosted version and now offering the software to install and run yourself

Meneame
Meneame is a Digg clone that has made their software open source so that anybody can create their own meneame, and many people have .

Scuttle
Open Source Del.icio.us clone.

New addition 15 November 2006:

ElggSpaces
From UK company Curverider who make the popular Educational software ELGG, they have launched their spaces box which is your own hosted fully-specced social network. Free (with ads) they then have 3 tiers of pricing. Domain mapping and full privacy controls are stand out features.