As a shareholder: not too worried about this. Instagram and WhatsApp have plenty of room for growth that can compensate for FB users leaving. A few weeks ago, WhatsApp released its official API[0] and at least here in Europe i have seen many companies starting to utilize it. For example airlines, sending WhatsApp messages keeping you updated on the status of your flight, much more convenient than installing a extra app for every airline. Lot of potential there.

[0] usable via third party providers, for example Twilio