A technique is described herein for providing implicit quality signals over a span of time that reflect quality of service provided by a collection of BOTs to a group of users. The technique can then leverage these implicit quality signals in various application-phase uses. In one use, an abandonment-determination component can leverage the implicit quality signals to provide an output result which indicates whether a current user has abandoned use of a current BOT with which he or she has been interacting, or is about to abandon use of that current BOT. In another use, a search engine or a recommendation engine can use the implicit quality signals to help identify an appropriate BOT for use by the current user. The implicit quality signals can include: one of more user-behavior implicit signals; one of more BOT-behavior implicit signals; and/or one of more transaction-summary implicit signals.