Indeed, although if you're going to use a "tor like service", why don't you just use tor? ;) Obviously not for everyone, but hosted services could easily set this up.
As for instant comms, I agree, and there's a balance of instant delivery vs pissing on an attacker's stats. The busier a network, the shorter the delivery delay can afford to be.
Additionally, the messaging service server could periodically ping hosts - if the whole exchange is encrypted and the server incorporates the padding protocol etc, it should make it harder to conclusively identify an individual exchange (although aggregated over time, statistical analysis is of course still possible).