Riak has been maintained through the post-basho years by engineers at some of its larger customers (disclaimer - including myself).
The focus has been on trying to improve the stability of the database when subject to complex failure scenarios under stressful load, with minimal need for urgent operator intervention. The focus has been on keeping those existing operators happy rather than seeking out new users. Evolution of the product since basho has been slow but significant.
The project now has support from Erlang Ecosystem Foundation, and we're looking to invest some effort over the next few months explaining what we've done, and to start to articulate what we see as the future for Riak. So if you're interested watch this space.
It is expected to remain a niche product though. However, it may still find a home for those demanding specific non-functional requirements, with an acceptance of some functional constraints.