This paper looks at how and why users categorise and curate content into collections online, using datasets containing nearly all the relevant activities from Pinterest.com during January 2013, and Last.fm in December 2012. In addition, a user survey of over 25 Pinterest and 250 Last.fm users is used to obtain insights into the motivations for content curation and corroborate results. The data reveal that curation tends to focus on items that may not rank highly in popularity and search rankings. Yet, curated items exhibit their own skewed popularity, with the top few items receiving most of the attention; indicative of a synchronised community. We distinguish structured curation by active categorisation from a more passive bookmarking by `liking' an item, and find the former more prevalent for popularly curated items. Likes, however, are initially accumulated at a faster pace. Finally, we study the social value of content curation and show that curators attract more followers with consistent activity, and diversity of interests. Interestingly, our user study indicates a divided opinion on the relevance of the social network.