StumbleUpon, Reddit, and Digg are social bookmarking sites that can drive traffic if used properly while contributing to the community. StumbleUpon focuses on serendipitous content discovery through algorithms and works best for images and videos. Reddit has user-run subcategories and requires being an active community member to avoid being seen as a spammer. Digg emphasizes connections with other users and requires continuous activity within a 24-48 hour window to maximize traffic.