Its always good to get different perspectives, in this case Dare Obasanjo (who, for the record works for Microsoft but is blogging his own opinion) provides a critical review of the OpenSocial API in comparison to Facebook:
"In general I believe that any effort to standardize the widget/gadget APIs exposed by various social networking sites and AJAX homepages (e.g. iGoogle, Netvibes, Live.com, etc) is a good thing... Given that Web widgets are now a known quantity, the time is ripe for some standardization.
That said, there are a number of things that give me cause to pause with regards to OpenSocial"
Its a technical perspective, but worth reading. Care of Brad Feld.