In a little less than 6 hours, Twitter will officially have 500 million registered users. This estimate is only slightly off from the projections made last month; on January 13, Twopcharts calculated that Twitter would hit the watermark on Valentine's Day. Being only eight days off still doesn't really change anything in this arbitrary speculation: Twitter is going to amass 500 million users. And that's a lot.
As of writing this, the total amount of registered users are 499,787,589 (the number continues to rise as I write, so we're just going to stick with that number for now) with roughly 11.7 accounts registered every second. If you're a Twitter user, you're probably acutely aware that spambots abound on Twitter so it's anybody's guess as to how many of those 500M users are actually humans. Similarly, there's no estimate on how many unique users there are but that would be a more difficult number to pull from the fog.
If the projection offered from Twopcharts remains steady throughout the day, Twitter will likely be popping the cork somewhere around 2:30PM EST. Then again, that's as long as Nicolas Sarkozy (or another political bugbears, for that matter) doesn't complain about too many more parody accounts and get them banned.
Yesterday, MediaBistro's AllTwitter predicted Twitter to hit 500M at 3:45PM, so the micro-blogging site seems to have picked up the pace since then. It could slow down, it could hasten - it's not like this is a derby race so when it happens doesn't matter. Well, I suppose it matters if Twitter ever files an IPO because people who gauge these things seem to be encouraged by huge user numbers. So I guess it does matter if you plan to get rich off of Twitter.
If you don't plan on getting rich off of Twitter, then I suppose these numbers just mean more spambots for you to block.