By far the best evidence is the ads one gets on Twitter now. I haven't been using it for about a month, but before I stopped, the ad deterioration was striking to the point of absurdity. It went from being like any other mainstream thing - sports or primetime on TV or google or instagram ads or whatever - to being the kind of stuff you get at 2am on local access channels. They struggled to have a competitive ad business for their size of audience before, and it could not be clear now that they are making absolute peanuts on ads now, and there's just no way subscription revenue is going to make up even a small sliver of that squandered advertising opportunity.