The Independent Sector does studies regarding on volunteerism in the USA, but not so much volunteer management. Still, check out their most recent studies.
Post your question to CYBERVPM -- that's the most popular volunteer management community in the USA, and if there is a more up-to-date study, someone there will know about it. Also try ARNOVA, an online community of academic researchers who examine volunteerism. If you re interested for other countries, try UKVPMs for Great Britain and OZVPM for Australia.