I almost got caught by a fake Twitter story a few days ago. It was pretty small-time as far as things go on a subject I'm not super familiar with (it was about the Brexit guy in the UK getting milkshaked,) but there was a solid few minutes where I thoroughly believed what I was seeing. It was only when I looked at the replies that I got more suspicious.

How many things do you see in a day, though? You're not always going to click around and make sure THIS THING is definitely true. If even 0.5% of the things you see on a daily basis are completely made up and you believe them, that adds up over time. It's way too easy to get fooled.