When I go to DL's Twitter page, it's clear that people are still reaching out to DL and DL is asking them to DM their PNR, etc. So, maybe DL is trying to spread out the workload across multiple platforms, including FB, Twitter, and now the DL chat (via the DL app)?
I've been in situations where I have Wifi but no cell data (or access to free text messaging) -- like when I'm traveling abroad. In these moments, being able to get in touch with DL for ticket changes or to check on something (such as getting a GUC or RUC correctly applied) without having to pay for text messages has been really helpful. I'm really, really hoping that DL Twitter isn't gone.