It seems current design cannot bypass the bottleneck from relationship, I think I need to re-think about the design.
It was said doing denormalization at the write time is a good way, as these nosql data stores are really fast on write. However, it may be too much to do it. My current scenario is every user has 10~100 buddies, if I do the denormalization during the write time, it still not clear to me how to set up a schema to fit it.
I found I’m still sort of in RDBMS world, need to jump into this nosql universe as soon as possible :P.