I don't think they are successors, just different. Protobuf is a sparse wire format, whereas FlatBuffers and Cap'n Proto use a fixed-layout wire format. There are plusses and minuses to both. I wrote a little bit about this here: https://news.ycombinator.com/item?id=6329041#6330426

(Disclosure: I work at Google on the protobuf team)

gRPC uses Protobuf version 3. Both it and CapnProto are successors to Protobuf version 2. Flatbuffers is not a successor but is targeted at a different use case.