Social media platforms should not be ad-supported.
This is just a continuation of that theme. He's saying, ~Your employees are doing asshole things to 3rd party developers, even though you're calling it a platform, because they have bad incentives, and that bad incentive is coming from the search for ad revenue~
Essentially, he's not so much mad at Mark, or even really caring whether Mark reads it or not. This is another piece of proof (to him and others that might listen), that social media platforms should not be ad-supported.
"The only way to get fulfillment is to help other people that believe what you believe." - Simon Sinek
You should watch the TED video where Simon Sinek talks about this. Dalton is basically putting this crazy idea out there about a non-ad supported social media platform (gasp!), and trying to find people who believes what he believes.
No good deed goes unpunished. Moreover, it's just not smart to publicly pick a fight with Facebook after they offered to acquire you. If you're the guys on the other side of the table, you might well brand this guy "unreasonable" and reckon that Kevin Systrom beat him once (score of $1B to $0), and Kevin is now playing for Facebook.
Seriously, this was not a good move on Dalton's part. As for this ("perhaps the public markets...will give you the time and goodwill to fix the obvious structural flaws"), he must be joking. Facebook has helicopter problems. Their dilemma is whether they are worth 20 billion dollars or 100 billion dollars. For Dalton to lecture Zuck in this tone, when Zuck has lapped him like 1 billion times over, and after they extended him a hand in good faith...just not smart.