I responded to you in your support ticket. Here's my answer from there:
Unfortunately the OAuth protocol is not supported in its cleanest form in Sitefinity and the ACS connection will not work out of the box. However, there is a chance that a custom implementation might do this. The first way for this to happen is to use the Sitefinity STS and to delegate through it the requests to ACS. The other way is to entirely skip the Sitefinity STS and call ACS directly, but in that case some of the Sitefinity handlers should be overriden and the login process should be customly implemented.
We are also working on an example that will show how to integrate Sitefinity with facebook login, so it might be helpful for you when it comes out.
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