1st case : In fact so many people consider or act as if anyboby is friend but i think they lost themselfs into the infinity of possibilities offered by social networking, they even add people for no reason (contact farm :-/). Your best interest is to act as normal life, would you act as a friend with someone is not ?

The quality of your network, depend on who you add into ;-)

2nd case : If you used to be friends and you lost contact by the time, why not contact them again ? it's just a new dynamic into your friendship thanks to the web ... but remember, the time is not all the time your friend, and people change sometime ...