It seems to me that the overall interest in Facebook is decreasing. The social network hasn't had any interesting feature added to it in the last couple of years. It's becoming boring and boring, so that's why I believe people are leaving.
Still, Instagram and WhatsApp are running strong with barely no competition. We don't see any news about their user base decreasing and news channels don't seem to dislike them. Facebook is doing a good job making sure their biggest three platforms are seem as independent from one another, keeping Instagram and WhatsApp almost free from controversy.
Personally I see no loss for them here. Besides, they will promptly acquire any new players that look promising, or shamelessly copy them as they did with Snapchat.