Reddit, Digg, and StumbleUpon are examples of popular

Question 18 options:

social networks.

tech blogs

video sites.

social bookmarking sites.

social bookmarking sites

Explanation:

Reddit, Digg and StumbleUpon are all Web sites dedicated to social bookmarking.

They fetch the contents from other Web sites and encourage their users to share their preferences on what they like and see all over the Internet with their friends.

They're not social networks per say because people aren't directly in contact with each other, only through their shared contents.

They're not tech blogs, because they rarely have any original contents and they're not targeting only technologies.

They're not video sites either, because they're sharing all kinds of contents.