In Diaspora as it exists now, replication - really, federation - is between pods. There's a protocol for transferring data between pods that is deliberately database-agnostic:

https://wiki.diasporafoundation.org/Federation_protocol_over...

So CouchDB's replication doesn't really help.

If the day comes that any single pod is big enough to need replication between clustered machines within it, then CouchDB should certainly be a contender for storing its data.