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.

> messages sent...

do you have a moment to talk about our lord and savior email :D

I don't disagree, but just because something isn't sold for >0 money doesn't mean it isn't in the market. Google is very much in the "search" market even if they do not make revenue directly from you the end customer making the search.

Giving something away for free and monetizing something else still means you are in that market.

> Facebook's "market" is advertising in which it isn't the leader.

What are the alternatives for social ads? Google is a different kind of ad service , kind of like billboards vs tv

Twitter, Quora, Reddit, LinkedIn, StackOverflow, Youtube, etc.

Google's display ad offerings may not be directly social, but their Adx and Adsense ads are shown across most forums and long-tail social channels, including Reddit and StackExchange. Lots of competitors in the social ad space.

Facebook is large, and might be the largest, but it's not fair to say that Google is an entirely different type of service. Hell, Google even offers native ads a la Facebook through their "Discover" ad product which serves across all their various social feed channels like Gmail, Youtube, and Discover feeds.