Like others say, it is a bug with the Facebook app. They do not manage the bluetooth keyboard right and if there is on on-screen keyboard, they remove the "send" button. The solution that works for me is the following:
1. Write my comment with the bluetooth keyboard until I am ready to send.
2. Push the "Eject' button to turn off the bluetooth keyboard.
3. Wait a second for the Send button to appear
4. Press Send to post the comment
5. Press "Eject" to turn bluetooth keyboard back on.
This has worked for me just fine and although it is slightly frustrating, it is not impossible to remember. After all, with the Bluetooth keyboard turned on, there is no "Send" button.