WordPress just hit an impressive milestone today: it has now filtered its 50 billionth piece of spam. Akismet, the homegrown comment filtering system from WordPress maker Automattic is the tool responsible for all the spam fighting. And those spammers keep the system busy. In April, Akismet blocked 1.8 billion spam messages, or 60 million pieces of spam per day, 2.5 million per hour, or 700 per second.

Whoa, that’s a lot of spam.

Akismet, those with long memories will recall, was the first product Automattic ever launched, arriving on October 25th, 2005 – a month before WordPress.com. WordPress sites now attract over 600 million unique visitors each month, according to Quantcast, and WordPress powers 1 in 2 blogs today (including yours truly). 50,000 to 100,000 new blogs launch on WordPress daily, giving spammers a seemingly never-ending network to target.

The Akismet filter works by combining information about the spam it captures on all participating blogs, then uses that info to define rules to block future spam. The system is now included by default in WordPress builds (since version 2.0) and in all the hosted WordPress.com blogs. There’s also an API for use outside of WordPress.

According to stats from the company, 92% of all comments checked by Akismet are spam. (Also, whoa!) Last month, the anti-spam system was used by 12 million sites, with an accuracy rate of 99.9%, Automattic claims. Which kind of makes it all the more impressive when you see a spam comment get through, when you stop and think about it.

Akismet’s last big milestone occurred in April of last year, when it filtered its 25 billionth piece of spam. At the time, it was detecting spam at a rate of 40 million comments per day. (As noted about, it’s now up to 60 million).