Better MongoDB Scalability Architecture ?

I'm working on ad service will host a lot of news website with a big traffic about 40 Milieon page view monthly , i was using mongodb with shards only without any replica and It was fitting very well . someday alot of issues with one of shards and every thing goes down. I lost configs metadata and resotre them again. so I rebuild the structure and hope if any one could tell me is there a better structure to serve my app smoothly and be easy to scale or this architructer is good enoght , what must I avoid to be in the safe area as mush as possible ?