As of yesterday, the favorites on Twitter are a thing of the past. Twitter and Vine have now replaced the favorites with Hearts instead and the're now called "Likes".
In a blog post on blog.twitter.com, Twitter say, "The heart, in contrast, is a universal symbol that resonates across languages, cultures, and time zones. The heart is more expressive, enabling you to convey a range of emotions and easily connect with people."
Twitter also ran some tests and they found... "And in our tests, we found that people loved it."
So, as of yesterday, the hearts have been rolled out across Twitter for Android and iOS and also on twitter.com and well as Vine.