Last month in my series on Twitter, I noted that according to one reporting site, "only a quarter million of (Twitter's 3 million registered users) post on any given day." As of today, Tweetrush notes that between 250,000 and 270,000 Twitterers are active in any given weekday. By that measure, Twitter is still very much a niche phenomenon given that (according to Nielsen) the active Internet audience is 372 million.

Popacular.com is hosting a Tweet counter called GigaTweet. According to it, the number of Tweets surpassed one billion recently. Sounds like a lot, but popular IM platforms handle more than this number each day. Again, this statistic makes it sound as if Twitter (and all microblogging) remains a Social Media service with narrow appeal.

But TechCrunch thinks Twitter is having its "hockey stick" moment--so called for when a site sees a sharp rise in visits and usage akin to the bend in a hockey stick. As noted by Erick Schonfeld:

Since January, Twitter has experienced a 16-fold growth in the U.S. And that is just visitors to Twitter.com. These numbers don’t count all the people who send and read Tweets from other Websites, desktop apps, or their mobile phones.

I suspect TechCrunch is correct--Twitter is in the process of going mainstream--but the microblog tool has a long way to go to get there. The chart below demonstrates the number of visitors to MySpace. Facebook, and Twitter. The Twitter line is so small it's hard to see the phenomenal growth, and according to Compete.net, Twitter traffic will need to grow 1500% to equal Facebook's traffic.

Twitter fans may object to this comparison and point out--with merit--that Twitter can and is used by many people who do not visit Twitter.com. Still, with the number of Twitter users south of 3.5 million, it's clear that Twitter won't achieve mainstream status for quite some time yet. I have no doubt it (or another microblogging tool) will do so, but it may be a bit premature to tag it "mainstream."