I wonder how much each user would have to be charged to at least break even. Of course if their business model changed to a pay scheme I imagine a large portion

I wonder how much each user would have to be charged to at least break even. Of course if their business model changed to a pay scheme I imagine a large portion of the user base would leave rather than stump up even a very modest fee. Is there room for a similar service that charges a one-off fee, recurring monthly charge or fee per message?