I was delighted last week to find a desktop client for Facebook Messenger rather than have a Safari window constantly minimised. However, today working from a nice little nature spot I sat down with 100% battery in my MBP. Tethered to the iPhone, I had Pages, Notability, iBooks and FB Messenger open. I was only typing in Notability of them all.
I replied to a FB message about an hour after setting up, and I spotted my battery was at 57%. 30 mins later, at 16%.
The fans were also spun up slowly, but I could feel heat rising through the keys venting out of the machine despite being in the shade on a nice breezy spring afternoon…
This is my call, I’m calling it now… there is something else going on in the Facebook Messenger app. There is no way a simple client like that could justifiably use so much battery power? I wouldn’t like to think so given that it’s on the official Mac App Store, but then we do hear horror stories every so often, and who’d be that surprised if Facebook was up to something…
Anybody else noticed this? Your thoughts on my tinfoil hat theory?
Facebook’s apps in the past have generally been known for not being fantastic platform citizens, so that wouldn’t exactly shock me.
The other thing to consider though is that the Mac is notoriously bad at “sipping data” when it’s tethered, and a lot of those services that are accessing the Internet are also doing other things (iCloud syncing / analyzing photos, etc.) that could potentially grind your battery.
Grab an app like Tripmode to make sure that the rest of your Mac isn’t accessing the Internet unawares.
Then you could do a controlled test with only that app open and watch your stats in Activity Monitor to see what’s going on. And if you see the behavior again, get Activity Monitor open right away and look at the “Energy” tab.
Just some thoughts for narrowing things down. 
The app has been built using Electron, a technology that helps make apps like this but is notoriously hungry for resources.
So, not that I’d ever vouch for FaceBook, that amount of battery drain is easily put down to the tethering and a sloppy Electron app, no malice required.
I would uninstall and stick to Safari when out and about if I were you. Avoid Chrome too if you can.