Given the amount of times I've had to "re-secure" my account settings, I think I'll pass on this one. At least I think/hope the data Google makes available is anonymous. When developers have access to the data (Farmville) you can be sure it's not.
Why You'll Give Up Gmail for Facebook Mail [Facebook]:
"Get ready for a new invasion wave from Facebook: Mail. According to Techcrunch's sources, a full webmail client integrated with The One and Only Social Network will debut next Monday. This is why it may become your favorite webmail service. More »"