Yahoo has never been successful in social networking type of sites, it launched a few and closed due to lack of interest among users. Flickr and Delicious are successful but they are not launched by Yahoo, they are acquires. The newest bet in this space is Yahoo Meme, a microblogging service which is more like a Tumblr clone than twitter.
Yahoo Meme is not new, it was launched in Portuguese few months ago quietly. After that they launched a Spanish version and now an English version of the site. So Yahoo Meme will become global pretty soon.
The service allows users to add text posts, upload photos, videos, music, others can see those updates much like Tumblr and Twitter. Users can follow others and repost stuff they posted in their memes, each post carries reposted link backs and comments.
Yahoo Meme is currently in Beta and there is no official information on when it will launch widely. If you want to use Yahoo meme you can add email address to the waiting list.