Here's a chart of Facebook's overall app score according to Applause Analytics over time (since 2011). It is an abstraction on the generic star rating.
What it shows is that there has been a general trend down in their app score - there seems to be a further dip around the time of Messenger but the reviews seem to indicate a lot of frustration around stability.
For apps that have been popular for a long time, I wonder if there is a negative bias in reviews (beyond that already present in all online reviews). A customer who gets an update that breaks an app they use every day is very likely to submit a negative review (even if they have reviewed positively in the past). However, a customer that gets and update that make incremental changes that largely don't affect them has no incentive to go post a positive review of the new version. So, the positive reviews come only from new customers, which are relatively small for apps that have reached saturation.
One interesting sub-trend in the data would support that view - it is that the review star rating, meaning reviews+star rating is, on average, more negative than just star rating. So people post a negative review when they feel pain, but not in the case where they are just rating the app.
I still don't understand why people hate Messenger so much. It works fine for me, indistinguishable from when it was included in the main Facebook app.
Messenger is much more intrusive to the whole phone experience than the Facebook messaging functionality that was included in the app. Though, to be honest, even that went downhill in the same direction as Messenger -- though not as far -- before it was removed and messenger made mandatory.
Many don't trust FB anymore or the invasive data tracking of Messenger. I've overheard comments from friends and family that they won't be forced to download another app, especially one that keeps such close watch on users.
http://i.imgur.com/iB2SMwC.png
What it shows is that there has been a general trend down in their app score - there seems to be a further dip around the time of Messenger but the reviews seem to indicate a lot of frustration around stability.