Facebook is tight-lipped about such matters. I do not think you're going to get confirmation here, (not one that I would trust).
Facebook is using both.
Re:Cassandra 0.5.0 released
by henrikschroder on Sunday March 21, 2010 @09:00PM
(#1208990)
Memcached is a completely different product, why would you stop using that just because you start using Cassandra?
Memcaheddb on the other hand is, IMHO, a misguided project. People ditching that is a very good thing. :-)
The argument is that memcached is unnecessary because of the speed of Cassandra, and the fact that .6 includes it's own, built in, row level cache.
Speaking of Cassandra, can anyone confirm that facebook is actually moving from Cassandra to hbase?