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Facebook Messenger on mobile, without having to download their messenger app
Pony Express lets you use Facebook's messenger on your phone without having to download Facebook's Messenger app.
If you don't want to download Facebook's Messenger App but still need to access Facebook Messenger, you can use Pony Express.
Pony Express is a simple app, it uses less battery power than Facebook's app, it doesn't ask for invasive permissions, and it doesn't bother you with notifications all the time.