Hey,
I personally, do not think that the internet leads to social isolation. The people I know who are on the PC quite a lot, also do go out and speak to friends around them all the time. I personally do so as well, and have friends both in and out of the internet, so am not socially isolated. It possibly could lead to social isolation in cases such as you being incredibly shy, or bullied or something - you may find that you are able to communicate more in the internet, so making you even more shy or not able to talk to people outside of the internet.
As a whole, as I said, I do not believe it does cause social isolation. It really depends on the type of person you are, as well. Many do not take talking on the internet as a big and important thing, but for others, it can become their life.
Hope this made sense and helps.
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