Myself and some guys with a telecoms background spoke to various telcos about that ~2 years ago. Feedback was that they were already so over stretched with data usage they weren't going to embark on any projects likely to increase it until there had been a lot of network upgrades. May have changed since then though.
I think the biggest risk in doing this for a VPN service is that the usage is impracticable given its nature and secrecy.
With services like Facebook, Twitter, etc, the telcos know more or less what they can expect in average data usage from users (and plan/charge accordingly).
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.
