I'm not sure about the order of these, and I don't have the hard numbers... but I've seen these folks spending in the Social Media space:
1. Advertising / Media
2. Games / Entertainment / Film
3. Consumer Electronics
4. Mobile Applications Development
5. General B2C Branding
Most industries will be following suit as soon as they start figuring out strategy. According to eMarketer, 88% of leading marketing execs report that it's "important" or "very important" to move into Social Media in 2010.
If you'd like more of my thoughts on the matter (from a previous corporate executive's perspective), rather than bore you with that stuff here, I've got a number of original video commentaries and some written posts about Social Media for Business at http://www.shakeitup.tv