When comparing Messenger vs Wally, the Slant community recommends Messenger for most people. In the question“What are the best Material Design apps?” Messenger is ranked 22nd while Wally is ranked 32nd. The most important reason people chose Messenger is:
Allows communicating with anyone who's a friend on Facebook.
Specs
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Pros

Pro Uses Facebook friends to create a contacts list
Allows communicating with anyone who's a friend on Facebook.
Pro Read receipts
Can see who has seen each message in group messages.
Pro Easy to share posts from Facebook
Integration with Facebook makes it easy to share any interesting posts from the Facebook timeline.
Pro Great UI
User interface of the application is clean, intuitive and easy to overview.
Pro Lots of different ways to find wallpapers
The app allows searching for wallpapers by text and tags as well as allows filtering them by categories, resolution, and/or aspect ratio.
Pro Has a huge selection of high quality wallpapers that's constantly updated
The app uses wallhaven.cc as the source for wallpapers. Wallhaven.cc, the successor of wallbase.cc, is a vast resource for high quality wallpapers.
Cons
Con Lack of privacy
Messenger is owned by Facebook. Based on their privacy policy, this means that any information you provide (including messages sent) are used by Facebook to profile you and sell access to your data to advertisers.
Con Lack of end-to-end encryption
Secret (end-to-end encrypted) chats are only supported in one-to-one messages, and are not enabled by default, and the encryption has not been verified independently.
Con app not available in play store
Con Buggy
Multiple people are reporting that the app can sometimes crash or crash the launcher or not work properly in some other way.
