Google's new OpenSocial API will allow developers to create applications for every major social network site except facebook.
A leaked press release revealed MySpace and Bebo are joining up with Google's OpenSocial API along with Six Apart, Orkut, Salesforce, LinkedIn, Ning, Hi5, Plaxo, Friendster, Viadeo and Oracle. Total uniques per month is 150 million while facebooks total uniques is 50 million.
OpenSocial API is like a form of web app syndication. Design it once through OpenSocial API and it will work through all of these social networks to a degree.
If you have a choice between building in OpenSocial's API - which so far has been reported as easier to use for 150 million and Facebook's 50 million harder to develop for - which are you going to choose
Well probably still both since 200 million is better than 150 million and Facebook's platform is still reasonably easy to develop in :).