I think snail-mail would suit the Steampunk community - the chance to use fountain or dip pens, wax seals, gold leaf, etc.
My experience with reddit.com/r/FountainPenPals was a lack of connection -- I had fun with it, but rarely did the correspondence continue beyond the first letter from each match.
Any advice for starting a new pen-pal matching service?
- limit of 2 pen-pals at a time 'till you've participated in a previous round of matching & nobody complained you flaked on mailing them
- offer InCharacter (writing as your steampunk persona) or OutOfCharacter (writing as yourself) options:
- Prompt for IC: "The Great Alliance is signed, your government calls on you to write these foreigners & get to know our new allies to strengthen the bonds among our nations"
- Prompt for OOC: you see your pen pal's answers from sign-up process questions, like "what's your favorite part of steampunk?" or "how do you participate in steampunk?"
Figured I'd use Google Forms to get everyone's address, email, one-off/continuing-correspondence preference, IC/OOC preference, & answers to any prompt questions -- leave that open for ~10 days, then email everyone their match(es).
I'll post the announcement on a few steampunk & pen pal forums like BrassGoggles, reddit's r/steampunk and r/penpals, the local steampunk Facebook, etc.
If it goes well, run a new round of match-ups every month or two.