1. It's not easier to use them than JSON when just getting started. However, the pay off is the strong typing and zero-copy access that they offer to folks that need to support clients on multiple architectures.

2. No, writers can directly embed structs and primitive data types into binary buffers at runtime through an API generated from an IDL file. Readers use direct memory access to pull values out of the buffers. If you set it up right, this can result in a massive perf boost by eliminating the encoding and decoding steps.

3. Facebook uses them in their mobile app. Another commenter mentioned use of the in the Arrow format. The flatbuffers website isn't the best, but clearly documents the flatbuffers IDL.