I once went on a tour with a stranger who answered my ad in a local bike club newsletter. We flew to the other side of the world and had a great time. There was some friction between us, but we ended on good terms and subsequently went on 2 more trips together.

I answered another cyclist's ad in the same club's newsletter. He was seeking touring advice rather than a touring partner. We started riding together and became friends. A few years later, we went on our first tour together and have been on many more since then.

I found another touring partner through an ad. We rode together for 2 or 3 weeks and then split up without bitterness.

I've had pretty good luck finding touring partners on the road, sometimes in very out-of-the-way places. I can recall one cyclist, however, who I met on the road and who started annoying me within a few minutes.