I'm sure I've missed one but I can't think of a dominant force in any of them fields that YC have invested in. I don't know about every investment, and like I said I've surely missed one, and I'm sure their portfolio has come closer than Microsoft to them markets, but if I were looking to find someone capitalising on them I can't think of any YC company immediately. Any ideas?

Edit: To be fair, Dropbox beat Microsoft to making cloud services popular. Microsoft do have a successful web services sector now though. Also I guess Reddit count for social, they have a very solid niche - but they wouldn't benefit under a company like Microsoft. They admitted it was a miracle Conde Nast didn't destroy their brand.