I don't follow. The VPN provider will still have to insist on limits, because the telco will insist on limits. It's not like Facebook is automatically unlimited, it's just limited to what Facebook is willing to pay for its users. It becomes Facebook's problem to understand data usage by its users and its apps and to pay accordingly (or not pay, and let the data start counting once the user is past the 'Facebook limit.')

Therefore, a VPN provider will be in the same position. So instead of paying AT&T for your data plan, you're basically paying the VPN people for your data plan plus privacy from AT&T.