Sometimes someone doing a search will stumble onto one photo or set, and they'll know other people who share their interest. They email two or three, who each email some others, and it can really spread.
In 2006 on my web site I posted photos from
Johnstown, PA. Apparently there's a very large community of Johnstown expatriates who still love their hometown and have fond memories and keep in touch, but never see photos anywhere. About two weeks after I posted the thread, my page views took off. They stayed above 200 for a couple of weeks, and the highest day was 1,400 views. I received a couple of dozen emails, many thanking me and sharing memories, and some offering additional commentary on some of the scenes. Only one bitched at me; "How could you not photograph such-and-such neighborhood?"
It was a one-time thing; nothing like it happened on my site before or since.