Rolf, good find. I duplicated this bug in the latest version (15.0.5962).
- In Inbox, type Facebook in search box (all Facebook messages are shown)
- Click on another folder (the Search box is now blank)
- Click the box at the top to select all messages in the folder.
- Click Delete and click OK in confirmation box (no messages in this folder get deleted)
- Go back to Inbox (all Facebook messages are gone)
Kevin