I'm a member of several LinkedIn discussion groups, but my participation is lukewarm at best.  One of the biggest problems is spam--the noise is drowning out the signal.  There doesn't seem to be a way for moderators to review messages before they're posted or for participants to flag spam for review by moderators.  As a result, a group I belong to that's dedicated to discussions of nonprofit social media projects includes the following off-topic messages:

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I'm a forum moderator on TechSoup and we hosts spend a lot of our time reviewing messages and deciding whether they're spam (many are spammy but not quite over the line).  It can be a lot of work, but it pays off in a forum that's mostly signal.  Part of the solution is having decent tools for managing spam.