read-only user
n. A person who uses the Internet exclusively for reading Web pages, e-mail, and newsgroups instead of creating their own content.

Example Citation:
"Until recently, the Internet relied almost exclusively on individuals to create content, so non-contributors weren't appreciated and came to be known, pejoratively, as 'read-only users'."

In Usenet circles, read-only users are more often referred to a "lurkers". This isn't strictly an insult because netiquette suggests that people spend some time lurking in a newsgroup before posting. After a while, however, regular group readers are expected to participate (that is, to "delurk") and failure to do so adds a negative connotation to lurker (that is, the person becomes a "mere lurker").