Siemens is a little extreme in my view. There is a time to lurk, and from the point of view of the group it can be useful for participants (especially new ones) to listen and reflect before participating actively. Peripheral participation can also enrich the discussion, bringing in other perspectives than those that are the central focus of the group. Lurking does not necessarily equate with taking, and endless activity, taking up peoples attention with your own views rather than listening to others may equally be construed as a kind of selfishness.
There are also questions related to people's comfort level in relation to participation. Some students post little but post well, and when forced to post more do not necessarily respond with "better" posts. Signal to noise ratio is an important issue.
But equally there is also a time to participate. When you are ready, when it is likely to be a useful contribution.