I have some thoughts, but no real answers... Today is my first time receiving an ID to send to someone. I am sure it has to do with my settings being "send only to the US"... Prior to today, every time I tried to send a surprise, I received an error message stating there were no participating members :(
I am an active member on Swap-bot and on SendSomething.net too- I seen this site mentioned on one of them (like you, on someone's profile page), and that's how I came across it.
So, when I am done posting this comment, I will go into my profile on both of those sites and add this one to it- that way others will see it and possibly join as well (They are obviously mail enthusiasts like us, so its the right target market!).
Short of that, and spreading the word through our current pen pal letters, I don't think there is much we can do to increase traffic...