The first step would be to come up with a model for the number of FTers on an AC flight. The obvious model, and likely close enough to correct, would be that the number of FTers on a flight follows what is called a Poisson distribution. This is because the number of seats on most planes is fairly large and the proportion of FTers among all travelers is fairly small. In any event, the extent to which this model is incorrect likely isn't going to be the biggest source of error in the final estimate.

The harder part is figuring out the "rate" parameter, which will be the chance that an individual AC passengers is a FTer. A rough first estimate could be the number of FTers who participate on the AC forum divided by the number of people who fly AC, say, in a given year.

What we'll never know, unless there is public data about this, is how many times the average person flies AC given that they fly AC at all, and the same for the average FTer in this forum.

You could guesstimate a range of possible segments per FTer (like, 4 to 20) and just take the average for everyone else at something reasonable like 2 to 4 (i.e. one or two round trips) and using all the information (or, "information" if you like) come up with a range of plausible values for the required rate.

Call the rate lambda, because we're using a Poisson distribution so calling the rate anything else would be heretical.

Then your probability of 1 or more FTers will simply be 1 - exp(n*lambda) where n is the number of seats on the plane.