Unbanked use prepaid visa cards to purchase things online. This just gets rid of the "tax" incurred when buying prepaid visa cards. Stat that would be interesting to see is smart phone penetration into this group of people.
Also unbanked can purchase amazon gift cards with cash.
They are growing really fast in the USA, mostly because the banking system here takes advantage of people much more than the banking system in the UK. Prepaid cards give people control over their financial lives, they are expensive but cheaper than overdraft fees and expensive credit lines. Growth is coming from younger demographics, so I'd say likely to keep coming: http://www.businessinsider.com/the-rise-of-reloadable-prepai...
Also unbanked can purchase amazon gift cards with cash.