I sell advertising for BitTorrent and am very close to people at Google, Facebook and Twitter who have told me that from a promotion standpoint (at least in the US) their lawyers see way too much potential liability at this point taking in advertising. A factor is that promoting a product or service from a distributed network may be hard to track down should there be an issue.
However as people have stated here all of the aforementioned companies will be incorporating crypto-currencies into their offerings and they really cannot stop people self promoting in their postings.
SHAMELESS PLUG : BitTorrent does accept advertising from crypto products and services. Our audience skews 74% male, and they are very much of the cord cutting, technophile mindset and already trust distributed computing. Contact me at worldwide@bittorent.com if interested for rates