It's not clear to me that the $4.99 or $9.99 per month will insulate you from redesigns or other updates that the company sees fit to deliver to their customers

It's not clear to me that the $4.99 or $9.99 per month will insulate you from redesigns or other updates that the company sees fit to deliver to their customers. If you're one customer in 1M, you're sunk if you represent a minority opposed to a particular change (such as the Gmail redesign). On the other hand, the ad-support argument makes sense - paying in cash rather than advertising attention for a service removes the tension between 'users' and 'customers' by making them the same person. Certainly a fee-for-service Facebook might optimize the site in completely different ways from the existing one - advertising impressions would not be a factor. However, as others have said, it would simply not be Facebook as we know it since it would have much, much fewer than the existing 750M users (1%? 10%?).