Facebook has an internal tool called thanksbot that you can use to “send thanks” to people, and these are recorded and can be discussed at performance review time. Ideally only in situation where it actually matters (thanks for the refactor that made our service use 50% less gCPU!) but obviously anything can be gamed.

I’d love to know as well. Thank you @rubidium for starting this comment tree and also @ping_pong for additional insights. Clarifications and/or corrections are totally welcome as this my comment is based on pure speculation.

It sounds like if employee A sends an email to employee B and employee B sends a response to A, employee B expects (implied or explicitly stated in their response) a "thank you for your time/response" email from employee A. Just a guess though.