1. There are merits of social interaction via the Internet.
2. There are merits of using the Internet for social interaction.
I would use 'to'.
1. There are merits to social interaction via the Internet.
2. There are merits to using the Internet for social interaction.
Of you could use so:
The merits of social interaction via the Internet are manifold.
Or you might say:
There are merits in interacting socially via the Internet.
As to the reasons, well better ask Ms Soup, my favourite expert!
But I think of it like this:
There are merits (which are attributed) to social interaction via Internet.
This is an opinion, not a certainty. The merits, as yet unexplained, are not necessarily 'of' 'social interaction via Internet' in any innate way. They are subjective, if existing, and so are attributed to it by people or persons. So I go with 'to'.
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