Google's spam filter is scary good. I don't think I've ever had anything accidentally marked as spam or had actual spam come through. The only unwanted messages are from services that I haven't changed my email preferences for yet.

They probably do.

We use a non-google spam filter at my office. And literally about 92% of the mail that comes is in junk and gets filtered. And this is a small environment.

When I worked for a University (50-60k users), our spam filters were closer to 98-99.5% filtering junk. And there still was a LOT that go through, namely targeting spear phishing attempts.

Google, with the traffic load they have, and how good their spam filters are (and they are good) I can totally see 3 nines in spam filtering... In fact im a little surprised its not closer to 4 nines.

Surprisingly, I've never received spam on gmail.