I don't understand that chart. What does "% change in millions" mean? Comparing the % change for a small user base (WeChat) to a much larger user base (Facebook

I don't understand that chart. What does "% change in millions" mean? Comparing the % change for a small user base (WeChat) to a much larger user base (Facebook) doesn't make much sense either. Facebook is huge, so a 60% growth in their mobile app is still impressive.