You may be confused at what Share is; it's a desktop application that uses the bittorrent protocol[1] to do P2P transfers between users. It integrates with Facebook for very much the same reasons as SendRail does.

See https://torrentfreak.com/bittorrent-releases-new-share-appli...

[1] It's actually developed by BitTorrent Inc, the same guys who write µTorrent.