What counts as an impression for your ad?

Social media sites charge a business based off the number of impressions an advertisement receives…but what is an impression? According to Facebook, an impression is any time an ad passes through a person’s screen, no matter how quickly. With many social media users today scrolling mindlessly through their newsfeed, it is likely that any “impressions” aren’t actually being seen. This is hurting businesses as they are being charged even though the ad scrolled past too quickly to be noticed. However, these social media platforms don’t see it that way. Instead, they are claiming that a consumer can notice and take in an entire ad in as little as a fraction of a second.

Is this true or are advertisers being scammed out of their money?