How To Design For One Billion Users

For folks curious about the number, AddThis is a widget that publishers use in their pages. So yes, the numbers are uniques, counted as accurately as it is possible to count such things given cookie deletion rates, etc. Here for example is builtwith's view of AddThis usage on pages relative to other third party widgets, like the FB Like button, to give you a sense: http://trends.builtwith.com/widgets

Alexa is probably very inaccurate for them because AddThis is a widget that wraps a number of social services into one little package. It's used on tons of websites.

The 1b may or may not include repeat visitors, they say both things on their website. Either way though, when you're integrated in other people's properties rather than trying to guide traffic to your own 1b gets a lot more possible - I'm almost up to 200m/month already.

As of last month, Comscore, who is in the business of measuring such things, puts AddThis at 57% of total world wide internet users (which they measure as 1.3 billion users). They put AddThis at 80% of total US internet users (which they measure as 212 million). Comscore's world wide penetration is not as deep historically as US, and is widely considered to undercount this #.