But I do agree, if Google had invested in FB they would have probably not pushed for OpenSocial so aggressively and so quickly. But I think they would have eventually done it, meaning they would not have been stopped by FB.
I think there is a lot more to get out of OpenSocial. If they can reach out to the whole web instead of one network they can probably do a lot more and target a lot more people. As they said it themselves, they look at the web as a platform instead of one site on it as the only platform.
So far with OpenSocial they have Myspace, Bebo, Orkut, Friendster, SixApart and +++ which is almost = to the web. This is not a bad deal compared to one site that is mostly about what it could be in the future rather than what it is actually right now (which is not much compared to Myspace, and definitely nothing close to all these sites combined.)
Google has its grudge against Facebook, microsoft has invested and created a high valuation and facebook potentially launching a ad service with better user preference knowledge than its old adsense platform
facebook is growing and has a mindshare of top 16-30 year olds, the youth whom every advertiser targets.
Facebook app development is not easy, no ajax, fancy UI for now, but as a innovator and a great developers dveloping facebook and facebook apps they can overcome those.
if google had to team with up everybody to compete against facebook and facebook application developers then facebook has been able to achieve what it intended to.
but over time expect opensocial to add other API besides current widgetting capabilities as they dont want to look like a cheap imitation.
also I suspect google would be writing some intermediate code for interfacing developer apps with other opensocial sites.
most of this communication would be via REST and JSON
for developers and development companies like ours we would put our bets on apps/sites which give us better revenues and development oppurtunities. we would continue with facebook apps and may port some to opensocial if demand exists
On top of all these it is a lot easier to develop for OpenSocial as it is for Facebook, since you can merely use HTML and Javascript to do so, instead of learning the FBML (facebook markup language). Although it should be noted that in order for a developer to do more customized stuff, they will need to work with the APIs of those sites that have one, this way they can take total advantage of the underlying websites.