Also, isn't a monthly active user of Facebook Messenger anyone who logs into Facebook?Of that list, I would be considered a 'monthly active user' of all but two.
That said, 99.9%(not sure how many 9's) of my messaging is done through Messages on my iPhone.
This list also makes my point. Who wants to keep track of which friends/family/colleagues/coworkers/contractors/service providers/etc use which messaging service? "Need to message Alice, uh, she's on Telegram ... now I need to message Bob, hmm, that's right, he's on WhatsApp".
Yes, those apps have huge subscriber counts. While I haven't done the research, I would hazard a guess that a very large percentage of actual regular users (ie. ones that use those third party apps every hour of every day) are Android users because SMS is insecure and limited in functionality.
That would make me a monthly active user of LinkedIn Messenger because I login to LinkedIn once a month or so and Messenger is always open down there.