According to the Google it self: "There are many websites implementing OpenSocial, including Engage.com, Friendster, hi5, Hyves, imeem, LinkedIn, MySpace, Ning, Oracle, orkut, Plaxo, Salesforce.com, Six Apart, Tianji, Viadeo, and XING."
It seems logical to me, that FB will follow the trend to adopt the OpenSocial and offer it as a tool of choice to the developers who are willing to use it. Facebook has already taken its steps to help developers by, for example, opening up its javascript library.
I recon adoption of OpenSocial would not hurt FB, but would make it even more attractive in the eyes of developers. Even though Microsoft is its shareholder.