Yammer's last venture round was in February of this year, raising $85 million at a rumored valuation of $500-$600 million, according to an article in TechCrunch. The February financing brought total venture investment in Yammer to $142 million. Assuming the rumored valuation range was correct, this is a nice (and fast) payday for the February round investors.
Venture capital investors in Yammer include Charles River, CrunchFund, DFJ Growth, Emergence Capital, Founders Fund, Khosla Ventures, Meritech and USVP.
Link to the WSJ.com story: http://online.wsj.com/article_email/SB10001424052702303822204577467312505454118-lMyQjAxMTAyMDEwNDExNDQyWj.html#articleTabs%3Darticle
Link to the TechCrunch story: