[stewartallen]

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 #.