Non religious sites are a no no as for facebook and Hi5, if your converation is in tuned with sabbath observance then cool. If you are just saying hi to friends etc then that's cool, the same way you would say hi to friends at church.
I agree with you, having conversations through the internet is the same as having a conversation face to face with the person or over the phone except one may be thousands of miles away. Technology allows us to shorten the distance barriers between us :)
A safe guide to 'what sites I may view' on sabbath is found in Isaiah ch 58. The principle to be extracted is this:- anything that detracts from my focus on the things of the Kingdom, is non-sabbatical and therefore should be excluded.
Consider what we are doing, at the moment, for an example.
The principle of Sabbath keeping is easy. Do that which draws you closer to Him. If your activity is not uplifting and helping you to focus on Him .... then don't do it. Communicating with Christian brothers and sisters on Facebook .... should be a good experience .... but only you can know and determine that. If the conversation is not spiritual ... then it would not be a good Sabbath experience.
May you be renewed In Him each Sabbath day ... and Happy Sabbath.