Today I was trying to determine which of my still-active online accounts is the oldest. The oldest I can reliably determine is eBay, which I apparently signed up to on Feb 12, 2000. I actually had an older account than that, to which I forgot my password. Whoops.
My Amazon and Wikipedia accounts are almost as old.
The actual oldest might be ICQ. I have a very low 7-digit UIN, which I reckon dates to some time in 1999. (Edwin has a 5-digit UIN. Showoff.)
But I reckon the winner must be my Yahoo account. Sadly, like ICQ, they don't tell you the date you created your account, so I'll probably never be able to figure it out for sure.