What if the fake traffic outnumbers real traffic by a wide margin, say 100 to 1 or even more?
If I'm completely honest, asking these questions is admitting you don't understand the subject matter well enough. People in general seem surprisingly naive what data collection and the subsequent processing entails. This is a wonderful quality, but also means companies can collect with impunity. This is an industry built on deducing marketable conclusions from huge amounts of raw and noisy data. It's far beyond some guy or basic algorithm looking at your browsing and going "you looked at a backpack, you must want a backpack". That happens too, but is a incredibly crude and clunky approach compared to the profiles built of people and their behaviour.
What's done is preferably collecting large amounts of data and matching patterns to other know patterns. Facebook can say with an uncomfortable degree of accuracy whether you're about to engage in or end a relationship. They don't do this by looking at the content of your messages, but at the metadata like how much messages you send when and similar patterns. Note that Facebook also collects large amounts of data about non members. This is also why large scale data collection is a dangerous game as you can make accurate predictions about things people aren't even aware of themselves. This includes illnesses, pregnancies and all kinds of medical data but also political matters. Remember Cambridge Analytica? It's industrialized manipulation. Other than on the micro level people are ridiculously predictable.
To create meaningful noise means creating an adversarial model. "The other side" not only has a massive headstart but is also ridiculously well funded as it's literally an industry worth many billions of dollars, so there's not really a chance of even putting up a fight. Note that I'm simplifying a couple of things here as it's a massive, sprawling and quickly evolving industry.