They also pissed all their page owners off with Edge Rank, which was rolled out too aggressively for my taste. Since not every facebook page owner is a facebook or SMM genius, a lot of small pages lost a lot of engagement, which turns into less engagement on the site overall (from users and page admins).
I started using G+ a couple of months ago because of their Hangouts, but am gradually becoming more engaged on the site. Facebook will fall, and pretty soon I think. Too much spam, horrible filters and crappy customization on your share settings make it a shitty service now. As G+ emphasizes share settings control, I can see why it will take over pretty fast, they just need to gain a little more momentum.