This paper examines the extent and rate of adoption of five Web 2.0 applications (RSS, blog, tagging, podcast, and wiki) in 111 American Research Association (ARL) libraries and identifies institutional factors associated with Web 2.0 adoption. The popularity of Web 2.0 continues to grow, both in terms of the total number of adopting libraries and the adoption rate. Wiki adopter libraries are in larger universities, have larger professional staff, and are more acceptable to electronic resources than the non-adopters.