Having it installed doesn't mean using it, like you said, for the PGP keys being generated but not necessarily in use.
On textsecure usage
In my experience, very very few people actually sign keys, and few people upload them on keyservers.
Also, I know very very few people who use textsecure. I used it for a few month, trying to get others to talk to me with it and gave up. And I'm in the bay area working with IT people, most of my friends are IT people mind you, we're exactly the people who try that stuff all the time.
It doesn't mean its bad tech or a bad attempt at all, heck, I would LOVE for it to catch up. But I seriously doubt it has 4 millions active users or more.
On PGP usage
From a key server perspective there are still a large number of keys being downloaded, refreshed and updated daily.
Let's look at Mozilla's stats and they just seem to peer with one low traffic guy and themselves... http://gpg.mozilla.org/pks/lookup?op=stats
that's 1k new keys a day, and about 500 updated a day. We're far from the 50k claim since it would reach that in 50 days no matter what.
Of course saying there's only, say a few millions PGP users doesnt mean AT ALL the same as 50k.
On misinformation
Thus I'm calling bullshit. Purposefully misrepresenting data (large order of magnitudes, at that) is a big minus in my book. Being well-known shouldn't mean you can tell people bullshit - if anything, you have a greater responsibility when you do.