YouTube [3] has hit a milestone. One billion active users a month, it announced [4] on its blog yesterday. That's one in seven of us on the planet watching something on the Google video site every 28 days or so.

The firm revealed a few more interesting factoids: One in every two people on the Internet visits YouTube, and every single firm in Ad Age's Top 100 Brands uses the site for its advertising campaigns. There are, however, some signs that YouTube may be moving toward a subscription model for parts of its network.

According to the Wall Street Journal [5], Robert Kyncl, the firm's VP, told reporters in L.A. that a paying model was important to create additional revenue streams for content creators. While getting people to "take out a credit card" might be difficult at first, "over time, a lot of people will figure it out."

YouTube's parent company, Google [6], however, gives no indication of its methodology when it comes to what constitutes a monthly active user. Facebook [7], you may remember, ran into some difficulties [8] with this issue last year, when it cited a MAU rate of around 845 million before its IPO. Many commentators questioned [9] how Facebook got [10] that figure, which is somewhere over a billion right now, as Facebook hit that milestone [11] some six months back.

Just to add a little bit of context, Instagram [12] has 90 million monthly active users [13], and Twitter [14]--which celebrates its 7th birthday today [15]--has around 200 million [16], as of December 2012.

[Image by Flickr user DavidErickson [17]]