How do big WordPress sites block spam

Hi,

I own a pretty huge WordPress website, and currently facing a huge spam comment problem.

I do have Akismet, and it does its job pretty well too. But let’s say it blocks 1k spam comments every day but still 100 spam comments get through without getting detected. Those 100 spam comments (which are now in approved comments list) are interspersed with 30 legit comments.

I weed out the spam comments from legit ones manually but it takes way too much time.

I was wondering what do some big WordPress powered sites like Smashing Magazine and Mashable use to deal with spam comments, that I’m sure they get by thousands every day? I don’t see them using captchas in their comment forms.

If I know, maybe I can employ such a solution too.

Thanks for your replies in advance.