Re: Tax isn't my strong point...
you use their services for free
As you point out, it's not free - there's a real cost involved. What is missing (and I doubt will ever be provided until GDPR really catches up with them) is an option to pay real cash and not be slurped.
they just show you ads and maybe use your data that (as long as you consent to it) to sell to others
If only that were what they do. If all they did was show you ads then fine, but they don't. Untargetted ads are worth little, the big money is in showing ads carefully matched to the data they've slurped on you. And there's never any element of consent - both Google and Facebook will slurp data on you without you ever having an account or ever consenting to them doing it.
One thing that's "irritating" me at the moment is the number of sites where they say "we use these third party cookies, if you want to stop them then you go to the third party and tell them to stop". Great, so to stop Faecesbook tracking you, you'd have to create an account on Faecesbook (because they'll ignore you otherwise) - but to do that you have to agree to their Ts&Cs which give them permission.
So how can you tax someone that provides a service for free, just because they then make money of you using their service for free?
Note that the money isn't made from people using their services (at least with Google) - Google makes money by showing adverts to you on every f***er elses' websites. So even if you have never ever, not even once, used any "free" Google service - they will still be making money from your data because of the sites you do visit showing ads that they get paid for.
For good measure, Google in particular is very good at using it's clout (particularly it's dominance in search) to take over any market it wants to. There were other mapping tools around, and some of them have managed to survive so far - but when Google started giving mapping away free* by using cross subsidies from it's massive marketing bis, they had an advantage over anyone else. So some other company could have a great idea - but because they don't have the backing of something the size of Google, there's no way they can both develop it and give it away free. Thanks to short sighted sheeple thinking "ooh, free", other offerings have tended to wither away, or limp on with little development, or stick to a niche commercial market - leaving Google to monopolise that market and then use it's dominance to a) push adverts to further it's own business, and b) keep any upstarts from succeeding. The USA has laws specifically to deal with this sort of behaviour - but a poor track record of actually doing too mush of practical use with those laws, c.f. Standard Oil, IBM, Microsoft who all avoided any significant penalty for blatant abuse of a dominant position in one market to give themselves a leg up in another.