Define "add value", because otherwise I'm laughing hard at this notion.
If you define it purely as "produce a product that people enjoy using or find useful", then sure, it adds value.
But at a higher level, asking if it's a net positive on society as a whole, and I'd say Twitter will never succeed at that. Social media has caused a breakdown in political discourse where people no longer seek to understand, but instead seek to "win", and Twitter's short message limit completely eliminated any possibility of actual conversations in favor of 140-character[0] zingers that are nothing more than ridiculous straw men. Nuance is a thing of the past.
[0] I know it's 280 now, but for the longest time it was 140, and I'm not sure the difference really matters. It's still incredibly limiting.