When comparing Messenger vs Campfire, the Slant community recommends Campfire for most people. In the question“What is the best team chat software?” Campfire is ranked 30th while Messenger is ranked 74th.
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 Supports optional 128-bit SSL encryption
Pro Guest access
You can invite people to join conversations even if they don't have a Campfire account. Campfire will generate a link to share and the guest will only need to enter their name to join the conversation.
Pro Easy conferencing
When you need to make a conference call, all you need to click is "Start A New Call" and Campfire instantly embeds a phone number and password code in the message thread.
Pro Mobile apps
Campfire supports only iOS, but third party apps can be used for Android. There are also other third party apps that build upon Campfire and extend its functionality.
Pro Embeds images
Images that are uploaded in the GIF, PNG, or JPEG formats are represented as thumbnails in the chat room.
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 Future of Campfire is uncertain
37signals have "changed direction" away from Campfire, focusing on Basecamp. They've ruled out shutting it down, but at this stage it's uncertain whether it'll be spun off as a separate company, sold off, or run in maintenance mode without any new features.
Con No private one-on-one conversation possibility
Con Does not show who's online
