For Bitcointalk, there's no official API (yet; one of the requirements for the new forum software is modular API access so here's hoping), though I've coded enough bots for interacting with this forum to know that it's possible. You could either run it while you're browsing (userscript (adds a tiny bit of overhead on each affected page load, inherently buggier than any other implementation) or browser plugin), have your computer turned on (desktop program; safest though limited to when the computer's turned on (obviously)) or constantly (running on a server; highest uptime though you'd have to either keep the session cookies or your login details on the server + you'd appear as if you're always online).
For Telegram, there seems to be a
developer API (and I've been seeing quite a few altcoin-related Telegram bots pop up within my section) so it probably wouldn't be too much of a hassle implementing an auto-responder.
For Facebook, there also seems to be an
API, though as a user previously mentioned, I'm not sure on what their policy on auto-responders is.