Google made a lot of noise when it (pre-)announced OpenSocial in early November. Its alliance with a lot of social networks were, supposedly, going to put some major pressure on Facebook. But announcing before you're live has its disadvantages, especially when your competitor licenses its platform structure to the sites you want in your "alliance". Today Bebo , the largest UK and third largest US social network, said it will launch a clone of Facebook Applications in addition to supporting OpenSocial.
Internet companies in general and social networks in particular might not like Facebook as a competitor, but neither do they like Google. As a third player, hedging your bets and supporting both, and making neither an automatic winner, makes sense.
Update: Or my take on this might be completely wrong.