When did blogs become professional news organisations and therefore have to adhere to standards of journalistic integrity?

They're just websites, anyone can put one up and say anything they like as long as they don't break the law.

The important part of "journalistic integrity" is "integrity". People are certainly free act without integrity in what they say, but that doesn't mean they should.

Yes, but they don't get to be called Journalists. They are just wankers with a keyboard.

Kind of like people commenting on social news sites?

Sort of - but with a responsibility to be fair and accurate. And on THIS site at least, folks tend to offer links, or have pertinent experiences.