On Sat, 01 January 2000, Ben Buxton wrote:

Looks like numerous networks went off the air over the year rollover due to fear and the like, and this caused a significant drop in the number of announcements:

The precise numbers will vary depending where your BGP view is in the worldwide Internet. You are correct. There was a steady decline in both number of prefixes and number of asns in the 24 hours before Midnight. The global routing table shrunk by 1000 routes before the hour before Midnight EST. In that hour, another 600 routes withdrew. The routes remained stable at that level for 5 hours, and then began to climb back to normal levels. About 30 ASNs vanished prior to the rollover.

From my view, the decline actually started on December 24. But it isn't really noticable until the last 24 hours. Since the graph of practically anything about Internet always shows an increase, a decrease is an interesting change.