Probably 0.001 percent of the riding population. There are people who ride bikes everyday to and from work who are not fiscally empowered to own computers. There are people who ride bikes who are aged in their 50s and 60 and 70s and beyond, and have never bothered to learn how to use a computer, and don't want to. There are kids under the age of 14 who are not eligible to be members of these forums.
The membership of BFs is only approx 130,000, drawn from an audience all around the world. The active membership is less than 20,000 - that is, only 13% are actively involved in discussing bike topics (yes, I know there is always a rolling list of guests, but they can't discuss advice).
Forums like this generally are the domain of the middle-class, white-collar IT-savvy up to the age of 40. The numbers even then would be skewed by the free access that college and university students have to computers.