There are certain statements that should make you wary of study findings.

People who x reported y is one of those phrases.

“people who stopped using Facebook for a week reported lower feelings of depression, anxiety, loneliness and social comparison,”

This is the same argument you see in cosmetic advertising as "Women who used this serum reported reduction in wrinkles"

If the study has evidence that people who x actually shows y, It would be irresponsible to not say that directly. Dropping to "people reported" seems like an admission that there was no measurable effect other than the influence of the researchers on the opinions of the subjects.

Mental state can be difficult in this respect because it is much harder to objectively measure internal states. the fact that it is harder to do, doesn't grant validity to subjective answers though.

I was once part of a study that did this. It was fascinating seeing something that appeared to have no effect being written up using both "people reported" and "significant" (meaning, not likely by chance, but implying a large effect to the casual reader).