Everyone I know still uses Facebook.
I think for most people the fact that violations of privacy happen so frequently (government data leaked, millions of credit card records stolen from online retailers, Clinton emails), and that the world hasn’t ended for them, makes concerns over Facebook sorta “meh”. Yes, people express outrage when the latest Facebook invasion is in the news, but then they go back to hanging out with friends and family on their timeline or in groups.
A 15 million decline is only one percent of Facebook’s total users. And it doesn’t look like investors are panicking: it’s stock price hit an all-time high of 190 this summer and is currently trading around 170.