Well for my 1000th post I said something along the lines of "Hooray its my 1000th post", but I didn't make that big of a deal out of it. I don't really have anything planned for any post milestones but perhaps I should think up some. 1234 is coming up...
As for in real life, I don't really get all that excited about some milestones either. Stuff like your 16th, 18th, and 21st birthdays I don't really get all that excited about. Yeah I can do certain thing once I've reached that age, but I don't really see that as a good reason to throw an extra large party. Stuff like graduating high school or college, now that's something worth celebrating. You earned it, you didn't just survive until that point (unless you went to a college where you literally had to survive for 4 years, in that case congratulations). Then of course I'm not that big on celebrating anything. All my recent birthdays have been marked by a cake and some of my friends hanging out. But nothing that big. We'd just litter the basement with soda cans and candy wrappers while we played video games all day.