Artist Trading Cards

Have you heard of ATC - artist trading cards?  It was new to me, so I started looking into it.  

So I had to Google, and Wikipedia says "Artist trading cards is a conceptual art project initiated by the Swiss artist M. Vänçi Stirnemann in 1997. He called it a Collaborative Cultural Performance. Artist trading cards are 2.5 by 3.5 inches in size, the same format as modern trading cards or playing cards."

Ok, so that's how it started.  But is this a big thing? Or relatively closed?  There is a group on Facebook with 14k members but then how does it work?  Trading via snail mail works for me!

Of course I had to give it a try, make some of my own!