At some point I got hold of Scott Yanoff's list of interesting Internet services (capitalizing Internet was still justifiable at the time) and learned about the Weather Underground, Archie, HPCWire, and this new thing called the "World Wide Web" — I started telnetting to a server at the University of Kansas where I could use Lynx, and it seemed pretty clear that this was going to be a big deal, because of how enormously much easier it was than downloading text files over FTP.
So not only have I wondered how many open FTP servers there are, my exploration of the internet pretty much started with a list of them.
Nowadays I occasionally look for FTP servers because they tend to be less of a pain in the ass for downloading stuff than HTTP servers — you can usually get a full list of what they have, and they never interrupt you with CAPTCHAs. It's kind of like a real-world "shibboleet" — I guess sometimes assholes push mandates for CAPTCHAs and whatnot on a company's technical folk, but they leave FTP open because the assholes don't know about it.
If you're wondering how many open HTTP servers there are, Netcraft does a pretty good monthly survey.