Threats rarely come from above, but most of the time from below. Small and flexible companies start with niches and keep improving their performance, until they become a threat to the established player. In this case we see the S-curve model playing out against Oracle.
These migrations indicate that after years of development, many open source or low-cost databases have now attained performance that is either roughly equivalent to parts of Oracle, such as Postgres, or have developed capabilities that while irrelevant to much of the database market, are far in advance of any technology operated by the Red Borg, such as MongoDB or Riak or Cassandra.
Personally I'm a fan of MongoDB and Redis. I also tried CouchDB, but didn't find it very active.