TV doesn't run ads to the extreme right margin of whatever else on screen you happen to be watching. If it did, TV advertising wouldn't be worth any money, either.
Sounds pretty similar to the network "Up next" ads that run along the bottom everytime a show cuts back from commercial or is about to end.
The difference would be that you are "already watching TV", in the same sense that Google had you "already searching for something", but this same logic applies so long as the ad being placed is for something on Facebook, which many of them are.
Facebook doesn't pause what you're doing right now to show you five minutes of ads. If it did, Facebook advertising wouldn't be worth any money, either.
a.) Not really relevant, the point is that TV shows that adverts can work even if they're not being shown somewhere where viewers are actively looking for something that might be advertised.
b.) There are plenty of websites out there which are profitable through advertising without the logic of "viewers came to the site to look for something". Maybe that's a better example. Again, you could argue that facebook adverts aren't identical to those found on most sites, but that doesn't change the fact that website advertising can (and often does) work.
Advertisements work when they get people's attention. Google advertisements get people's attention because they're relevant to what the user is looking for, TV advertisements get people's attention because they take up the whole screen and are often funny or dramatic in themselves, Groupon emails get people's attention because they're actionable coupons that users really want to get, and "GOLDENPALACE.COM" painted on the back of a streaker at a professional sporting event gets people's attention because, hey, a streaker! Facebook ads are virtually hidden in a margin on the edge of the screen where users are more or less trained to ignore things.
I completely agree that Facebooks adverts are very badly implemented.
However, that doesn't take away from the fact that it is factually wrong to say that adverts don't work on websites where the users aren't already looking for what the adverts are promoting.
The difference would be that you are "already watching TV", in the same sense that Google had you "already searching for something", but this same logic applies so long as the ad being placed is for something on Facebook, which many of them are.