What significance does the level of participation have to an online community and what effect do lurkers have?

For an online community to thrive, a significant proportion of its members must participate. Otherwise the site will fail to offer fresh material and ultimately traffic will slow. Participation can be a challenge though; most users are lurkers: they absorb content others post but they don't usually contribute their own. Researchers estimate that only one percent of a typical community's users regularly participate and another nine percent do so only intermittently. The remaining 90% just observe what's on the site, so they don't add a lot of value-other than adding to the number of "eyeballs" the site can claim when it tries to convince advertisers to buy space.
How can a site convert lurkers into active users? The easier it is to participate, the more likely it is that the community can generate activity among a larger proportion of visitors. In part this means ensuring that there are several ways to participate that vary in terms of their ease of use. Facebook is an example of an online community that has figured out how to offer several forms of participation. Members can post status updates (very easy), make comments, upload pictures, share notes and links, play social games, answer quizzes, decorate their profiles, upload videos, and create events (a bit harder), among other forms of participation.