Imho, "lurking," pertaining to Cyberspace, has evolved to mean observing w/o being noticed, something of a "fly on the wall." Cyberspace makes it very easy to enter a community sight unseen, to gather information about the community flavor. In my narrower definition, one type of "lurker" is someone who wants to check out the community (newsgroups, listservs). Do the posters talk about things I'm interested in? Can I get information w/o sounding like I didn't research first? Is the atmosphere friendly vs. uncivilized?
Other "lurkers," imho, are ones who just prefer to read and absorb but don't say anything. That's their preference, and that's fine.
I myself don't use "lurker" as a derogatory term. Just my 2¢.
Wanda