Yahoo bungled Flickr and Upcoming by neglect - limited resources and demanding integration rather than features. I'd imagine Delicious was the same, but I don't have a link handy.

For Flickr: https://gizmodo.com/5910223/how-yahoo-killed-flickr-and-lost...

For Upcoming: https://medium.com/message/diary-of-a-corporate-sellout-5874...