In my rails 3 app I use Omniauth for the user authentication part (fb/twitter).
Actually I follow this:
https://github.com/RailsApps/rails3-mongoid-omniauth
Please make sure the cookie policy that your rails app follows does have sensible settings for your use case (see the link in my comment above). All I can imagine right now (knowing what I know, sitting where I sit) is that the cookie(s) ha(s/ve) properties that are suboptimal/undesirable in your context.
Please check the cookie settings in a browser debug/development tool such as firebug, firecookie or the chrome development tools.
Sorry, that's all I can come up with given my knowledge of the problem. Feel free to contact me again with more details on your cookie- and testing-setup.
My 2Cents.