Put it to you this way, there was a stretch in the late 70's early 80's where they averaged over 40,000 per game at Giants Stadium.
I've looked at the attendances, but I've also heard people from Seattle say that the Sounders still aren't as mainstream as we may think (as in, far behind the Mariners) so I wasn't sure.
Peaked at a 47,856 average at Giants Stadium in 1978.
Could you place them among the other NY sports teams as a measure of how popular they were? Not necessarily ordered, but who they were ahead of/behind.
NFL - New York Football Giants, New York Jets
MLB - New York Mets, New York Yankees
NBA - New Jersey Nets, New York Knickerbockers
NHL - New York Islanders, New York Rangers
NASL - New York Cosmos