We have similar experience to Howie's regarding 'All Company' Yammer postings.

Anyone can post to the 'All Company' group, but there seems to be a natural restraint, with almost no frivolous messages. Anything really inappropriate would be quickly removed, though there is no active moderation otherwise.

We have plenty of subject specific and location specific groups which do seem to attract more of the non-business related messages.

Perhaps more of a challenge is for the 'All Company' group to be used for truly important company-wide messages, rather than messages which may seem important to the writer but which are in fact merely 'nice-to-know'.