A couple months ago my cousin posted on Facebook about some postcards and stuff she received. I thought it would be nice to get something in the mail; so I joined
Back years ago when we used to go to Tennessee (USA) every year, I’d print out photos while there and send them as postcards to relatives telling about our trip
I discovered Postcrossing through youtube in 2018. One of my favourite Russian booktube blogers told about her hobby and I joined right away. Before that I only used to exchange letters.
I’m not sure anymore. I first joined in 2015 but I never sent any cards (!) so only recently rejoined this year. I’ve been collecting postcards ever since I was a teenager (in the pre-digital age) so I’m sure I must of found out about Postcrossing through some sort of postcard search.
I discovered postcrossing because of my teacher in primary school. We used to do postcrossing with the whole class. It was so much fun. Our techer read all the postcards for us and translated them. She sent all of the cards.
When the year was over, the postcards were shared amongst everyone!
I feel deeply grateful for this experience, it’s the reason why I have a hobby that nourish my soul now!
I learned of Postcrossing from YouTube years ago and joined only in 2021 because I was curious. The YouTuber was German but living in the Netherlands and it sounded interesting and her postcards were beautiful.
I saw a description of Postcrossing, with a link included, on a Facebook page for finding penpals. 
I joined that day!
Maybe this will surprise you.I found Postcrossing from a college entrance examination English test paper. This reading details the website! The English test of the Chinese college entrance examination features reading questions on various subjects. Thanks for this test paper. I’m grateful to finally be here!
My daughter used Postcrossing for a school project. The class loved it! She carries on as a participant, then her husband joined. She got her granddad to join! When she made me a Grandma at the beginning of COVID lockdown, I painted postcards for my new granddaughter and used them to document her life events and what was happening in the world that she was not allowed to explore. Now she is 3 and I still paint her postcards but now I also send them via Postcrossing. 
I was becoming increasingly jaded and unhappy with the postcard groups on Yahoo. The cards were fine; the members were a quite different story. Then someone mentioned “Postcrossing.” And here I am 12 years later.
I first heard about Postcrossing after stumbling across The Writerie’s Etsy page. I didn’t join until months later when an artist (Arcanum Dice) I follow on Instagram posted about their experiences.
Learned about PostCrossing a few years ago via a professional development workshop for teachers taught by Lisa Van Gemert, where it was mentioned as a possible classroom activity. Thought it sounded interesting and filed it away for future reference. Decided to give it a try once I went on my teaching hiatus.
I learned about Postcrossing when I was purchasing postcards at an artist’s table at a summer Art Fair. She excitedly told me all about Postcrossing and wrote out how to connect. I was engaged, at the time, sending regular postcards to my dementia friends. That was a fun lively project that blossomed especially when we got locked down during COVID. I also regularly mail postcards to my grandchildren (as a sort of history lesson on ways to communicate other than social media!). Sometimes, they sent postcards in return! I search out odd colorful postcards that are enjoyable to send and receive.
I wish I could remember! But I did join on October 1 2011, World Postcard Day, so I suspect it was via some sort of ad or promotion. Ironically, I’ve never achieved the WPD badge on my profile! 
October always seems to be one of those months I’ve fallen away from Postcrossing for a bit (I always come back!)
But this year I have plans to finally achieve that WPD badge 
I was active on the website “Where’s George” a currency tracking program. At the bottom of their home page was a link called “Other sites of interest”. I clicked on it and there was a list of various currency tracking sites. At the bottom of the list was a small link described as a postcard exchange club. I clicked on it and was immediately captivated that was 13 years ago.
I heard about Postcrossing from a local postage stamp dealer I had purchased vintage stamps from. He invited me to a letter/postcard writing event and mentioned it there! Several of the other attendees also piped up about how wonderful it is
I was sitting at my parents breakfast table the 1. may 2021. Reading some advertisements about vacation and started complaining about corona and that I couldn’t go on vacation last year and can’t go this year yet because of restrictions. And I explained that worst of all is, that I can’t sent postcards from vacation.
Well my parents were not impressed, maybe rolled their eyes…
So I googled “send postcards” and happily ended up with postcrossing 
I was watching tv, and in one quiz-tv show there was a girl who was talking about Postcrossing as her hobby. I created account the next day.
Printed on a plastic(?) bag, in Köln.
I read about Postcrossing in the Bookcrossing forum.
I got an e-mail from Deutsche Post, in which they write about postcrossing. And a week later i signed up.