Social bookmarking allows users to save and share bookmarks online. Users can organize bookmarks with tags and make them public or private. While this helps users collaborate and find information more easily, social bookmarking systems rely on user-generated tags which can be ambiguous or incorrectly used. There are also concerns about spamming and manipulation of tags and rankings. Popular social bookmarking sites include Delicious, Digg, Reddit, and StumbleUpon.