Similar problem here, but a bit more severe:
if w3 total cache and wp-facebook-connect is activated, somehow my user's session is being served to random visitors, making them able to comment as my user and access the admin control panel (NOT GOOD)
wp-facebook-connect apparently sets a fbs_<FBAPIKEY> cookie
i've set this cookie in page cache, to avoid caching, then loggen on with a fb user in a different browser, to see what happens.
If I connect with facebook, i get logged in, but looking in the page source, i can se this:
<!-- Performance optimized by W3 Total Cache. Learn more: http://www.w3-edge.com/wordpress-plugins/
Minified using disk
Page Caching using memcached (cookie is rejected)
Database Caching using memcached (user is logged in)
Object Caching 941/981 objects using memcached
so page cache is still on, but i'm logged in. (Don't cache pages for logged in users is enabled) What to do now?