"Hi, I'm a Mac."
"And I'm a PC. Wow, you suck, Mac. What the hell happened?"
"Yo momma, PC."
<wild gesticulating and arguing ensues for 20-30 seconds>
"Hi, I'm Linux. Neither of these people care anything about you. You see, you're not their customer anymore. When you're ready to make computing personal again, check us out."
The ad should show something people want, not vague promises of being their customer or personal computing (a term essentially unknown by the new generations). Show something the new machine can do that the competition can't - built-in adblocker, cross-compatibility with Mac and Windows apps via VMs/rented servers, etc.
The price of renting a billboard isn't going to cover more than a week's worth of those people's fees. Billboard-induced shame has actually much more chance of succeeding.
The whole point of FOSS is that a single groups decision or opinion do not dictated your computing. There are enough forks or alternatives.
It's also hard to measure the quantity and genuineness of bitching online because people complain about everything and there's an inherent incentive online to complain to bring in ad revenue regardless of how genuine it is.
But it's a direct and unmistakeable sign (to you and your peers and colleagues) when someone paid actual money to rent a billboard just to remind you how much you fucked up.