I can anecdotally echo the same sentiments as @StefanKarpinski with respect to nanomsg and other ZeroMQ alternatives being, for all intents and purposes, technological dead-ends. There are alternative transports, but they all exist at slightly different layers than ZeroMQ. For example, AMQP, STOMP, ActiveMQ, etc. provide message distribution semantics that can be implemented in ZeroMQ, but all require significantly more supporting infrastructure than ZeroMQ (or have other limitations such as being JVM-only).
In my experience, ZeroMQ has really found a sweet-spot between raw TCP/UDP and application messaging protocols. The ZeroMQ protocol has also been extensively documented and there is a robust RFC process in place for reviewing and accepting changes to it. Speaking of which, before he died, Pieter Hintjens created the “Collective Code Construction Contract” to guide future ZeroMQ development in his absence.
If there’s any appearance of a decrease in activity around ZeroMQ development, I think that reflects more the fact that it is maturing as a protocol and less that it is becoming, in any way, irrelevant…at least, that’s been my experience.