Is that a bad thing? Regardless of the fraction, if something really does cause more harm than benefit, we will by definition be better off without it, no?
If something causes more harm than good, then we would be better off in a hypothetical world where it was magically erased from existence. In the real world, the negative effects of trying to forcibly ban it can easily be worse. See: drugs.
They also don't account for the fact that Facebook usage is correlated with screen usage, which is correlated with happiness, so the effect they measured could just be the effect of screen use.
"We found that teens who spent more time seeing their friends in person, exercising, playing sports, attending religious services, reading or even doing homework were happier. However, teens who spent more time on the internet, playing computer games, on social media, texting, using video chat or watching TV were less happy.
In other words, every activity that didn’t involve a screen was linked to more happiness, and every activity that involved a screen was linked to less happiness. The differences were considerable: Teens who spent more than five hours a day online were twice as likely to be unhappy as those who spent less than an hour a day." - https://www.vice.com/en_us/article/kzn9d3/cell-phones-linked...
