My impression was that TokuDB is a single-server database, as a backend for MySQL. In that case it wouldn't be a direct competitor to HBase and Cassandra, both of which you normally wouldn't even consider unless you expect to have a cluster of machines.

You'd arguably be taking a risk by choosing a centralized database for write-heavy applications, which are the hardest to scale, but I could see it as a good fit for the applications where people currently use sharded MySQL.