Further, many Facebook applications are not able to take advantage form the Facebook platform now. The platform paid off well for the early adopters and also for the most popular apps, but the majority of applications on Facebook are not seeing much return from the site. Whereas, through OpenSocial they are now able to reach out to a much larger audience, that is also a lot more diversified with a lot more itnerests.
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.
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.