[ximeng]

Didn't downvote, but they could just as well take 20% of the bond as a fee.

Facebook could also charge $1 and give 50c to the user for receiving the message rather than keeping the whole dollar.

[niggler]

"otherwise the message is received without a cost to the sender"

Sure FB could take a cut of each message fee but that wasn't the OP proposal

[ximeng]

No, but it is compatible with the motivation behind it.