What is OpenSocial?

OpenSocial is an alliance of different network sites collaborated by Google to tackle the competition from Facebook. The API shall give developers access to:

Profile Information
Friends Information
Activities

Sites that are part of OpenSocial include:

Orkut
LinkedIn
hi5
Friendster
Plaxo
Ning
Oracle
Saleforce

OpenSocial is expected to bring together 100 million users, almost double the number of Facebook users. The major applications developers for Facebook have been invited to create apps for OpenSocial