What effects does spending a lot of time in online communities have on interpersonal skills? There are quite a few users who are pretty much addicted to the Stack Exchange network and spend over 3-4 hours per day here. Time spent here will result in lower amount of social interactions, and lesser time for other activities as well. 

What are the effects that this has on the social skills of a person ?

In a way, we are all interacting with other individuals on this platform as well, but the communications are mostly textual and in a way, buffered (for lack of a better term). So, its safe to say that spending half an hour interacting with individuals here will be very different from interacting with individuals face-to-face. In such a scenario, what are expected and/or observed effects on communication abilities.
There seems to be a difference between social community users and non-users (e.g. http://dx.doi.org/10.1016/j.chb.2013.01.026), but these are only correlative data, not causative, but in a recent study "Facebook usage was found to interact with measures of psychological well-being, suggesting that it might be beneficial to students experiencing low self-esteem and low life satisfaction." (http://dx.doi.org/10.1080/0144929X.2010.550063)

This is just a quick hint and doesn't comprehensively answer your question, but maybe the articles contain references to other research.

Follow the links to get to the original publications and a short abstract describing the study and results.



More stuff I found through a Google Scholar search for "online communities effects":

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC411092/
http://repository.maestra.net/valutazione/MaterialeSarti/articoli/Johnson-Survey.pdf
etc.

I don't know, if any of this is relevant, I just want to give you ideas to do your own research.

If you access Google Scholar through a university network, you'll get links to most of the articles, otherwise you'll still be able to read most of the abstracts.