1. Facebook
While there is facebook.com, there is ALSO an app from the Microsoft store--like there is from the App Store--for Facebook, so technically using the term "app" is acceptable.
2. @Jasper2009
GoogleHangouts may provide what you're after in the form of a group chat. I personally don't use it so can't speak to how long a group chat would remain "alive" or retain past responses, but it meets your requirements.
For something more robust, go with a bulletin board, much like this one here on FT. They can be had & hosted for free, and you can lock down access with usernames/passwords.