While it's nice that this gets some attention and not very nice of facebook to automatically revert you back to an unencrypted connection, this is not a facebook specific problem. Anytime you use a wireless network, where you don't have control over the access point, you need to secure everything you want to keep private. This goes for everything from google searches and files transfers to instant messaging and e-mail. The proven solution is to use a VPN tunnel, which even many home routers support nowadays.

Of course there's still a bigger problem with arp spoofing and other attacks, which in the long term will need to be solved. Maybe with something like DNSSEC DKI.