You could do members search - show all - sort by date registered, and start clicking on profiles to see who's still active. But apparently viewing all members is temporarily disabled.

Theymos probably has the highest activity here, there were only 34 people registered before him, his uid is 35. But possibly there are few older accounts still around. There's SmokeTooMuch, registered Dec 2009, last active 3 weeks ago, but his last post is from Jan 2016, so hard to say whether he's still active.