I personally believe that there is a significant number of seronegative MG patients who are not diagnosed as MG (and therefore are not part of the statistics).

When you assume that a certain combination of tests virtually rules out a certain diagnosis (when in fact the sensitivity of those tests is unknown), there is no way you can know how many patients truly suffer from the given illness.

Possibly you are right that seronegative (as opposed to seropositive) are over-represented in internet forums, but at the same time those who are less proactive and told they are suffering from psychiatric problems, or other " waste-basket diagnoses" (when in fact they have MG) are under-represented.

It is very hard to know what is the true number of neglected MG patients.
I have been trying to find this out over the last few years, without much success.

The patients we are seeing on those forums may the proactive edge of a very large ice-berg. The neruology community is overall very narrow minded and easily dismisses patients with myasthenic symptoms who do not fit the book.
Some of those patients may eventually (years later) be diagnosed as suffering from myasthenia, and probably many are not.