My theory is that for a developer, a LinkedIn profile is less useful because it basically just states who you know and what you focus on. Twitter accounts show nothing other than that you exist. You likely want to hide your Facebook - my boss was not my friend until after he hired me ;)
A github account is so much more than a blog. A great one need not have many repositories, nor many followers. It just needs to show that you:
- Can Program
- Document your code
- Use version control well (or at all)
- Love to program
A github account is an easy way for non-designers and non-frontend developers to have a portfolio of their past work. It's also a great way to exercise your mind outside of work.
Nota bene: Make sure you legally can write open source. YMMV, but state law might actually back that silly form you signed when you were hired. It helps when your open source code has nothing to do with your actual job.