I'm seeing the same problem here. My application was initially throwing intermittent 500 errors but a retry usually got around that, however, since at least November 27th has been throwing consistent 401 Invalid Signature. The code hasn't changed and after an extensive debugging trawl, the code is fine - the function generating the OAuth Signature is working fine when getting the Access Token, so it shouldn't be an invalid signature when making the request against the Social API endpoint.
It would be nice if this could be resolved quickly. It's at least a week since the 401s became constant.