Who kept their letters from old pen pals and what's the story there?

I've kept all my letters, but have been unable to find her on social media platforms. I grew up going to a small private school. We had missionary visitors come through that lived in RVs. Me and one of the girls my age (about 9) got along great, and played constantly for weeks. When it was time for them to move on to the next town, we were both very sad and exchanged addresses. Hers was a PO Box they frequently check and update when they move locations. We kept in contact for 10 years, keeping up with each other as we grew up. Then suddenly we lost contact? Very disappointing, and I always wonder what happened to her.