Despite the decreasing numbers of Facebook, I think the real problem is the way advertising has been used on Social Networks. Advertisers need to understand that SNS are cousy, intimate digital places in which people can hang out without being INTERRUPTED by ads (even if they are "social ads"). Everybody loves benefits, but very, very few people likes advertising.
I believe all advertising actions should be based on the UHU Effect (as I like to call it).
Utility: brands MUST be USEFUL in order to remain RELEVANT.
Honesty: Come on!! People knows best!! Trying to reach teenagers writing as one is not allowed!!!
Usability: Dah!!
So, numbers may go down (in terms of captive audience), but the real power behind SNS is AFFINITY, not numerical REACH.