This paper uses an Activity Theory framework to explore adult user activities and informal learning processes as reflected in their blogs and social network sites (SNS). Using the assumption that a web-based space is an activity system in which learning occurs, typical features of the components were investigated and each activity system then characterized. Data obtained from individual users of a blog (Naver) and a SNS (Cyworld) were analyzed at the individual level and extended into the social level. Based on the findings, commonalities and differences between blog and SNS activities were discussed and different types of the division of labor between the activity systems were identified. This paper furthermore suggests three dimensions of adult informal learning activities in Web 2.0 in terms of learning as (a) an acquisition process, (b) a reflection process, and (c) a practice-based community process. Therefore, its discussion helps advance the understanding of the potential value of using Web 2.0 applications for adult informal learning. (Contains 6 figures.)