Cash Now will never be the size of ICE....however, being the much smaller fish in a very small pond is ok too. While there are plenty of private Forex companies, there aren't any other than ICE that I know of that are in the public arena. Therefore, once they release their Beta you will see a big surge of new investors looking to profit from the ground floor.
I don't know the rule of thumb, but at some threshold shorts who play pinks must back off when the number of investors get too big. Some quoted 800 right now....up from a few hundred I think from December. The leverage of alot of small buyers is too much pressure (and risk) for a few big shorters to overcome....at that point we will only be trading amongst ourselves.