Systems and methods for communication of user comments over a computer network are provided. Users may place and review comments on webpages or other resources without the resources necessarily expressly incorporating such commenting systems and methods. User comments may be associated with particular elements of resources (e.g. links, videos, pictures, text, HTML tags). Comments may be displayed to the user based on the position of their associated elements. Commenting systems and methods according to the present disclosure may flexibly associate comments with particular elements based on metadata such as one or more attributes of an element, so as to account for changes in resources over time. Users may review comments left by other users, and users' view of other users' comments may be restricted based on visibility settings. User activity profiles comprising displays of user activity across multiple unrelated resources may be provided.