Instagram, Snapchat, Whatsapp and Reddit are the "in" social media sites. The definition is a bit lose given snapchat/whatsapp are more messenger services but Facebook is in decline. I complained in another thread about XF's social media strategy but it does seriously need looking at.
Exactly why I'd like to see this. My site is about a local amusement park. I wear a t-shirt with our URL each visit to the park. Thousands of young people see my advertising each time. I have heard so many times by many that they don't join "because it's just one more site to log into/remember." I think having an Instagram log in feature would null that excuse.
However popular Instagram and others might be, I'm not sure it's accurate to claim that Facebook is "in decline," and certainly not any kind of permanent decline. Despite dropping a million users at the beginning of the year, it's gained them all back and more. It added 48 million this past quarter, from what I found by searching online.