Facebook has a display network similar to DoubleClick, called Audience Network. In my experience it pays publishers a lot better than DoubleClick. For advertisers, they're able to target on more demographic categories (for now, the CA scandal has done a lot to reduce targeting options).
A significant chunk of that ~4 Billion or ~5% of total global online advertising is because Google also owns YouTube, which captures a large chunk total advertising spending independent of search.
Twitter also has an ad product called "Twitter Display" in which your promoted tweets get reformatted into display ads and shown in other ad-supported mobile apps. It was some of the cheapest, but worst-performing, traffic we've ever driven through a paid channel.
I've seen lots of money wasted on blogspam websites within GDN (non-retargeting) -- there's a need to be very hands on if you don't want to waste money across all display tactics, in my opinion