Social media usage is still quite new and there have been no major longitudinal study that I know of, that has looked into how usage influences social abilities in the long run. If you limit it to youtube only, then I don't know of any study at all that looked at youtube usage only and social skills.
One of the reasons it's difficult to study this, is that people who are drawn to these online activities are not the same as others, and not everyone who uses them becomes "addicted" to them. In other words, we don't start with people having the exact same social or cognitive skills. In addition, people don't actually use them the exact same way (length of time, content, etc). Not to mention that people often don't just use one particular thing like youtube but many different sites and so that's another reason it's difficult to make comparisons.
Of course, there are ways to use them that can be of benefit. For instance, concerning younger people who use social media including youtube, The American Academic of Pediatrics mentions potential benefits, such as communicating and socializing, better learning opportunities, and access to health info, while warning of dangers such as cyberbullying, sexting, violation of privacy, powerful influence of advertisement, and last but not least, "facebook depression."
An interesting area of study is "internet addiction". The final decision was made not to include it in the latest psychiatric diagnostic manual (DSM-V), partly because as one article asks "Problematic Internet use: a distinct disorder, a manifestation of an underlying psychopathology, or a troublesome behaviour? Other authors however have a specific definition in mind, such as this editorial that states the four defining criteria of internet addiction:
1) excessive use, often associated with a loss of sense of time or a
neglect of basic drives, 2) withdrawal, including feelings of anger,
tension, and/or depression when the computer is inaccessible, 3)
tolerance, including the need for better computer equipment, more
software, or more hours of use, and 4) negative repercussions,
including arguments, lying, poor achievement, social isolation, and
In summary: There is a lack of good longitudinal research in this very important area. There is also problem with defining what is appropriate usage and what is not. But it is speculated that proper usage (e.g. access to useful info, or socializing) can be beneficial while inappropriate usage (e.g. online bullying) can be detrimental to mental health. Regardless of purpose of usage, excessive usage to the point of ignoring more important daily functions may also be a sign of mental illness, be it caused by the computer usage itself or as manifestation of another already present illness.
Hope this has been useful to you.
Block J. J. (2008). Issues for DSM-V: Internet Addiction. American Journal of Psychiatry; 165, 306–307.
O’Keeffe, G., & Clarke-Pearson, K. (2011). Clinical report the impact of social media on children, adolescents, and families. Pediatrics, 127, 800–804.
Starcevic, V. (2010). Problematic internet use: A distinct disorder,
a manifestation of an underlying psychopathology, or a troublesome behaviour?
World Psychiatry, 9, 92.