I have heard that US citizens need to report each and every cryptocurrency transaction made by them in the income tax return, as per the directive from the IRS.
Correct. The form in question is Form 8949, which you can view here -
https://www.irs.gov/pub/irs-pdf/f8949.pdf - unless you have a 1099-B from your exchange(s) which is unlikely. Every trade you make, even if trading bitcoin for an altcoin or vice versa, needs to have a cost basis worked out in USD and reported on this form.
Now how many out of 50 million could have done this? I strongly suspect that a majority of the low volume users might have ignored this directive.
I agree. Further, given the number of exchanges which have scammed, been hacked in to insolvency, shut down, etc., not only will many people not have done this, but many people will be unable to do this retrospectively as they did not keep their own records and can no longer access their exchange's records.
And I don't think that the IRS will go after them for $100 or $200 in pending taxes.
Who knows for sure, but if they find out you have a KYCed account on a bitcoin exchange and you have never paid taxes related to bitcoin, then expect to be investigated.