Except with their mobile apps, where they separated Messenger from the main Facebook app, and everybody threw an enormous fit for reasons I'm still not really clear on.
(well, it's not really a clean separation since you still need a Facebook account and all that entails, but I still don't see how separating the two apps is anything other than a good thing)
For me it was one more application that's constantly pinging my location has access to all my data, and is actively feeding it upstream. Though I did keep hangouts (google voice and sms integrations), after facebook messenger and seeing my battery not make it through a full day (morning until I plug in at night), I removed pretty much all the social apps from my phone.
When I looked at my battery usage stats, the top offenders were facebook and the like, despite not even being used for days at a time. Now I use facebook through the mobile-web interface, and haven't looked back. The only apps I regularly use on my phone are the browser, mail, maps and hangouts. I don't have many others even installed (lastpass, authy), and my phone now makes it a full 24hrs+ before power goes off. (just got a new phone yesterday, so that profile may be different now).
My fit wasn't separating messenger out... it was having another app soaking up cpu/battery usage.
Yeah at first the separation of Messenger was something I was irritated and confused by, but I soon realised it was actually a very smart idea, as it moves to replace WhatsApp, SMS, and any other messaging service in a way that was never possible with Messenger just built into the Facebook app.
(well, it's not really a clean separation since you still need a Facebook account and all that entails, but I still don't see how separating the two apps is anything other than a good thing)