SLSB can only be in SL scope. When you make your facade a application scope component you have to use @Synchronized to have injection working with multiple clients. This configuration creates a bottleneck in this beans. The same applies with Session scoped components if you have pull ajax requests.
The recommendation is to have facades as SLSB. That scale better but my personal choice is to have a SFSB Controller backed by SLSB faces/repositories/daos. This way the SFSB will alleviate your DB was some list/beans/VOs would not need to be searched every time.
Hope it helps.