From the BBC:

Social network sites are moving to make it much easier for software developers to write add-ons for the hugely popular web destinations.

Bebo, Facebook, Meebo and Friendster have unveiled plans to help them become more than places to keep in touch.

The add-ons will allow users to add extras, such as video and music clips, to the personal profiles they maintain. ...

Significantly, Bebo's interface tools will work with Facebook's already announced development system. This will make it possible for the many developers who have written applications for Facebook to use their code almost unchanged for the Bebo network.

Despite the tie-up on tools, Bebo and Facebook will not become a unified network.

SOCIAL SITES: DAILY VISITORS
MySpace - 29 million
Facebook - 15 million
Friendster - 5.9 million
Orkut - 9.6 million
Bebo - 4.8 million
Source: ComScore July 2007
Bebo said it would also support Google's Open Social initiative which aims to create a unified system of tools that can be used on any and every social network site. The Open Social tools are due to appear in early 2008.

Social networking giant MySpace is backing Google's initiative