I'll write a longer post about this topic in the future. But here are two important aspects to consider: the globalization of these services, and the percentages of non-English-speaking users on both sites.
Consider the following:
- 75% of all Twitter users come from outside the U.S.
- 70% of all YouTube traffic comes from outside the U.S.
I tend to find, though, (especially on YouTube), that the audience is increasingly international and non-English-speaking (or quasi-English-speaking). I have 1,200+ subscribers on YouTube. Over half of them are from non-English-speaking corners of the world.
This is a factor to consider if your primary reason for being on social media is the marketing of your books. Certain kinds of content (like fiction, for example) are extremely difficult to sell to readers whose primary language is not English.
On Blogger, by contrast, 90%+ of my traffic comes from the USA, Canada, or the UK.
I think that the language barrier is the reason behind this difference. If you have a minimal grasp of English, you can still utilize YouTube or Twitter. However, an advanced reading level is required to read a blog.