Yep, I had a lot of fun with that one but it died a death very quickly. I wouldn’t blame the postal service, personally. I just stopped getting responses after a letter or two.
I don’t think we had too many players but I do think that putting us into groups of three or four initially was actually a good idea. I also enjoyed the monthly paper but we never got past the initial stage of getting used to things. So we never used the classifieds effectively, for instance.
The GM’s initial estimate/target of one letter per month per person you were corresponding with was about realistic and probably about as much as I could handle. It didn’t take me all that long to write the letters themselves but I found I spent quite a bit of time thinking about where to take things in the long term. I also did a bit of research into the era, which I enjoyed a lot. Any excuse for a trip to Wikipedia, really!
There were no NPCs other than those the players made up by mentioning them in their letters. So we could pretty much do to them what we wanted, although I remember being cautious about how much I made up about someone that hadn’t been introduced by myself. In other words, I tried to keep it interesting while avoiding the worst curveballs. (I don’t know if I succeeded… But that was the intent anyway.) Genre tropes are your friend here, using and/or subverting them makes for great entertainment.
My recommendation for running a game like that: find a small group of players that you know are reliable and start with them. Keeping it running requires effort from all sides and it’s very frustrating if even one person in your inner circle just stops responding to anything. Keep track of who sent what, and when. Consider giving a gentle prod to a player that’s more than a couple of weeks behind.
You as the GM are going to be the only person that has a full picture of the plot threads at any given time. Use that to everyone’s advantage.
Maybe less important but could be extremely beneficial: find players with an interest in or experience with actual writing. The letters may have looked a bit random but the real fun for me was trying to craft a story out of what I had come up with and what I was given by the others. I was really looking forward to having my journo go from sceptic to stark raving mad over the course of a year. It’s just a shame it didn’t happen…