It's not the internet, it's screens. Whether TV, computer, phone, tablet, or else, watching a screen regularly for an extended period of time has a significant effect on humans who are social animals.

There is enough of scientific data and studies over the last few decades on the tv screen to reach a scientific consensus but commercial interests and public relations have prevented this consensus from reaching the general public. we have less data for videogames and internet and smartphone but it's already shaping towards something similar.

And there is more than simply replacing actual human interactions by screens, for example most flat screens emit light frequencies in the blue range that have effects on the body and mind, impacting sleep and linked to macular degeneration.

Though that being said, facebook due to its format and content has a significant tendency in making people feel bad about themselves.