A quick Google search shows that iMessage does have the prerequisite 100 million active users.
With some quick searches, multiplying iPhone's 39% marketshare with 328million Americans gets me to 127million, but that's probably too high - not everyone has a smartphone, and how many users actually use iMessage at least once a month, and not say, WhatsApp exclusively ?
But the question was if there are a 100 million MAU in the US, let alone how to find that information in a simple google search. I couldn't find any breakout of iMessage users per country