Actually, "Founder" on the site and ingame are two entirely different things.
On POTCO, members who paid for their accounts before Dec 1st, 2007 at 11:59PM PST were rewarded with a special crew kit as talked about in
this thread and a golden name tag, representing that they were some of the first members to play the game.
On POF, members with the founder tag are the first 5,000 members to join PiratesOnlineForums.com. These members, as a reward for helping to get the site up on it's two legs, were rewarded with lifetime access to a free userpage and signature. Everyone after the first 5,000 members now recieve the "Pirate" tag instead.