As of now, around 30 per cent of the videos on YouTube have been converted to the WebM format. To speed up the process of transcoding, Google has been utilising its famed cloud infrastructure, whenever they are free. Whenever there is too much traffic or users need to upload content, the WebM transcoding is paused.
But it isn't like Google is totally abandoning the H.264 format as it plans to continue it in the near time to come. H.264 is also more popular since its offers hardware acceleration which is still non-existent in the WebM format.