[Petronski]

By adding a name to the "to:" reply box, the post turns up in that person's list of "Pings," alerting them to the message sent.

It's a way to alert another member of a certain thread or post.

As for its origin, I can only suspect it is from "Hunt for Red October," when Ryan is communicating with Ramius via morse code and tells Ramius to reply with a sonar ping:

Captain Ramius: Re-verify our range to target... one ping only.
Capt. Vasili Borodin: Captain, I - I - I just...
Captain Ramius: Give me a ping, Vasili. One ping only, please.
Capt. Vasili Borodin: Aye, Captain.

[posterchild]

As for its origin, I can only suspect it is from "Hunt for Red October," when Ryan is communicating with Ramius via morse code and tells Ramius to reply with a sonar ping:
I believe the term does originate with submarine pings, but its common internet use predates Hunt for Red October (1990) and is a unix command. If you have Mac OS, Linux, or any flavor of unix, issue a command line; ping www.freerepublic.com and you will get a report on network connectivity between your node and that of freerepublic.