One needs not "sell" anything to have a market share. For example, Google's market share in general search services can be defined as the number of queries made

One needs not "sell" anything to have a market share. For example, Google's market share in general search services can be defined as the number of queries made on Google as a share of all queries made on general search engines. Facebook's market share in messaging could be defined in terms of messages sent. That would of course be difficult to measure, but nonetheless that is still a framework to think about market share in messaging.