I always use & preffer Facebook. Facebook is online social networking site. Here you can chat with others, shared photos, videos, post yours thoughts by blogging, can do shopping, selling your products, can post your business advertisment, etc.
But Yahoo is an Internet portal, a search engine. Its a directory of World Wide Web sites. There you can read millions of world wide news, can see videos, images and many more. You can also shopping through Yahoo. Yahoo also have mailing features. It is an oldest portal or search engine.
at the end the real fact is that Facebook is most popular websites or social networking sites among in the world than other sites. The number of users of Facebook has crossed billion.