Google Apps? Windows Live?
Could you be more specific? All users are in the same organization (same domain) or do they want seperate domains/servers? Can it be outsourced or do you want to host all servers/services yourself?
It is really hard to tell that CouchDB was the wrong option, since (if I understand correctly) you're doubting your choice on performance issues.
Performance (or lack thereof) can come from the back-end as well, such as complex algorithms. Then there's the connection between the database and the application, caching etc.
I have tried cassandra for a while, but my knowledge of MySQL is so far superior that I stopped bothering with NoSQL and went back to SQL.