Instagram’s ad unit is excellent, especially compared to both Tumblr and Twitter. There’s no reason these platforms aren’t better at this. The problem with social media is not ads, but clearly there aren’t many teams that succeed at building a good mobile ad product.
This is true as a user of the core social products, not someone buying ads. But it’s pretty clear to me that ads that are good from a user experience translates to ads that are valuable to ad buyers.