When I looked at an analysis of my blog traffic, I found an unknown site "Diigo" in one of the traffic sources. What's Diigo?

Diigo is a research tool and a social content site. When I read a web page, I can highlight some lines and add sticky notes on the page. Then I save the page and share the page with everyone. It looks good.

I am always careful about private settings because most web services have a social network feature. Sometimes my private information might be open to everyone on the Internet without notice. So, first of all, I set the all private mode on. A few days later, I will change the settings. I want to keep my privacy.