Because they would be inclined to offer a better service as their competitors? They'd have to make more effort to please users. My guess is this would mean show less ads.

Independent Instagram or WhatsApp could push FB to reduce its reliance on non-original posting, could move into privacy as a competitive advantage, could steal new users, could steal people leaving FB. As it is currently, FB can herd users into using its other apps without improving its core business. They can be dodgy because people "leaving" them are just returning to them, if they go to Insta or WhatsApp.

What all the other posters said. If Facebook had real competition then it’s likely they wouldn’t be soaring in the stock market while in the midst of so much controversy and negative attention. It’s more likely that their competition would be pouncing on them by offering more distinct alternatives, forcing Facebook to either reform or fade away.

To state the obvious, because abuse (of users privacy, etc.) would then have a bigger negative impact on the company (FB) which would be an incentive to improve this behavior?