Why Are You Seeing So Many Bad Digital Ads Now?

Portia Kapraun has always seen unwelcome ads on Twitter, usually from major brands pitching her luxury jewelry or vehicles that she, as a librarian in Indiana, could not afford.

But the mix now is far more annoying: More ads for random gold investments, she said, and also a badly designed ad for what looked like a tabletop foosball set constructed with rubber bands and particle board, which promised its product would be the most fun family game she had ever played.

Ms. Kapraun was not interested. But she soon saw...