But what about when no exit nodes are involved, when you connect to hidden services? And if your underlying connection is properly encrypted, then the most that could be divuldged is metadata. You'd have to believe the U.S. government has also broken popular crypto, and I believe that if that were the case we would've never seen things like Dual EC DRGB. Even powerful quantum computers could only weaken cryptographic algorithms, if my understanding of Shor's algorithm is accurate, so you could always use paranoid-sized keys or post-quantum cryptographic algorithms.
The folks that work on and support Tor are by all accounts some fairly intelligent people. It is obviously very possible the whole network is compromised, but it would be very impressive if it was compromised and nobody ever found out.
I think software like Tor-based IM is still useful even in the event that the network is compromised to some degree.