Because, why would they in the first place? How would blockchain technology help Facebook and Twitter's platform exactly?
Remove your mindset of that every single thing should have it's own blockchain. Blockchain is very unnecessary in most platforms, hence why we see a lot of useless coins and tokens.
Basically this. Facebook and Twitter may be two of the biggest social networking websites today, but they don't need blockchain in the first place. They're good on their own. And I agree. Not everything should run on and use blockchain. It's a great technology but we don't have to shove it down everyone else's throats.