People seem to have a preference for celebrating "big round numbers", like the 2,000 post in the forum (although the 10,000 post would probably be more "special" - but not the 10,234 post, so it's not strictly a magnitude bias). There are lots of examples of this. Sports stats are another: baseball players celebrate 3,000 hits, 500 home runs, and 20 wins. Football players 1,000 yard seasons, and so on.
Why do we prefer marking events that are round numbers?