In regard of the revenue model i want to point out for what UpWork (one of the top global freelancing platforms) have done to there Service Fees.
They have been already charging 10% fee from every and each payment for any job you finish.
However, they still saw that as an Unfair rate for them!, so they Doubled that for any job you have with a new client (where their payment processing is very valuable), and reduced that to the half for the Much more regular clients of yours (where you may be tempted not to use their platform at all and just trust your client).
That what services for centralized and huge companies worth, we are much more vulnerable and deal with much more risks and offering a very valuable product.