How about just sending a message and chatting a bit? If you've only talked to a person for a minute, and it was years ago, why (how?) would you consider them a friend? There isn't much purpose of being "friends" on a site (unless it lets you see eachothers profile & pictures); all it really lets you do is have a list of people on one page when you want to send a message to somebody.
I recently got an invite to linkedin via email. The guy's name sounded very familiar but I had no idea who it was, even after looking at pictures. It took about 2 months before I figured out who this guy from a different state was and why he had my email. If you run into somebody on the street you haven't seen or heard from in years, would you ask them "want to be friends?" or would you say "hello, how's it going, what's new?" first?
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