edwest211 noted the article from National Catholic Register. Over 500 sounds like a lot until it is put in perspective; there are about 17,200 parishes in the US; so that puts the EF at about 2.9%.
What other information is known? A good bit of information can be culled from Coalition in Support of Ecclesia Dei. They list the days and times of the EF by diocese, with each parish listed. For example, in the Birmingham diocese, OUr Lady Help of Christians in Huntsville has 2 EF Masses on Sunday, daily Mass at different times, and 2 listed on Holy Days.
In the Mobile Archdiocese, St Bartholomew has the EF on the 1st Sunday of the month at 12:30 p.m.
The list stretches from parishes which appear to be EF parishes, to one example of the EF being offered 6 times a year.
There have been a number of posts in the past indicating full churches, but without any information as to the size of the church (and having attended an EF at a small church which holds perhaps 150 people, it clearly makes a difference in how many are attending). Additionally, without any actual head count broken down by age, it is not much more than speculation as to how many “young people” are attending.