I think it would be nice if users from Facebook, Twitter, Google+ could comment to posts using their accounts.
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- Any Stack Exchange user can ask a question
- The community provides support, votes on ideas, and reports bugs
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Commenting on any posts is a privilege that only users with a reputation of at least 50 (on Stack Overflow) have.
If you are referring to users with an account on Stack Exchange and a social network, I don't see the reason why they should use that account instead of the one they already have on Stack Exchange.
Users can already log in with a Google, Yahoo, and a Facebook account, as many other OpenID accounts. Doing so will create a new Stack Exchange account linked to your OpenID account.