There are several disadvantages of using the app instead of loading Facebook directly in a browser:
1 – The Facebook app is a major battery hog. After I deleted it from my phone I immediately started enjoying an extra hour or two of battery life between charges.
2 – Facebook’s mobile website works a lot more like the full-blown desktop site, making it easier to switch back and forth between the two than switching to the app and back.
3 – The app seems to be subject to more strange glitches than the mobile Facebook website. Of course glitches will still rear their ugly heads when using Facebook in a mobile browser, but they will likely occur much less frequently.
4 – The app seems to be less protective of a user’s privacy than the mobile version of the Facebook website.