My point was that Facebook don't also provide email services. Perhaps lock-in is too strong a term, but it's certainly convenient enough to discourage using anything other than gmail - and means that google will own a much larger chunk of your online identity than Facebook does now.
> My point was that Facebook don't also provide email services.
Incorrect, as of a few months ago. Every(?) Facebook user now has an email address: <userID>@facebook.com. You can send messages to external email addresses from the messages tab.