They should have a verification process for accounts beyond verifying an email address. Verify identity, it's way too easy for people to be impersonated, there's a whole TV show dedicated to this...
Facebook 100% reads your "encrypted" WhatsApp messages. I had a conversation with someone about a very unique topic on WhatsApp, 5 minutes later I see remarketing ads on Facebook about the same topic.
They are entrenched in enterprise. When all your systems (sales, marketing, ERP, finances, support, HR etc) are wired up to Salesforce workflows and automation, it's hard to leave the platform.
I agree, it will be hard to disrupt them through technology right now, but in the next 2-3 years there will be a new wave of tech they might not adapt to immediately.
Another possibility is them getting acquired, as they have already raised 7 rounds of funding (investor pressure), and their founder has had a large exit already (Flickr to Yahoo).