Something I'm trying out as of late (official site). As with any new hobby, we'll see if I stick to it :).
My current routine is buying numerous postcards in person, stamping them (on the back) with a single tourist stamp if the place has any, then picking one out of the pile to write on and send once I request an address and read the recipient's profile. If I really vibe with some of their interests and have related stickers, I add one as well! :) (artist's alleys at cons often have neat ones, throwing a sheet into the shopping cart at a merch store has been another source. Remains to be seen where else I can find good ones, but in general I'm happy to have a use for stickers beyond my Wanderbook diary..)
If you postcross as well and want to do a direct swap, send me a message. I use poste restante, so I'm not too worried about my privacy. For direct swaps, I might even look for a specific theme postcard for you, but no guarantees.