Let's not forget that the primary demographic in which any new trend starts is the college-age to twenties crowd.
Facebook smartly captured this specific demographic and their attention (away from Friendster and MySpace), because of two main reasons, whichi Google+ does not have:
1) social acceptance (ie friend confirmation button)
2) being the "first" network in which people could feel unstalkerish by stalking people they barely know but would possibly like to know better (flirtation, becoming friends, etc)
1) it started off being an "in-network-only" - what does that mean? It means, the college students which were its first users, mostly wanted to check out those hot girl/guys in their classes. It also had a "confirm friend", so you gained some sort of "acceptance" that it was consensual "stalking".
Google+, however, misses that boat: anyone can add you, without your consent (you can only block, not force them to unfriend). That means there is no "confirmed acceptance", missing out on a key social-emotional facet.
2) Furthermore, Facebook has most momentum *not* because it has "all your friend", but because it has "all the cool/hot girl/guys you'd like to be better friends with but-only-met-once-at-a-party-and-do-not-want-to-overtly-add-on-another-network-again". If google+ finds a way to migrate this set over to G+, I'd wager the G+ snowball would start rolling, and rolling pretty fast.
As an example, I was one of the first on facebook. So was my circle. But guess which same circle is on my G+ ? That's right! The geeky circle I have that was first on these due to being in "ivies" and having "friends who work at google". However, which group is missing from my G+ and everyone else? Those acquaintances you met once and never met again? Some you unfriend, but some you still want to see as a contact.
1) Facebook's confirm friend button works far better to make users feel safe
2) Facebook has snowball effect due to people having already added, surprisingly, not their EXISTING friends but rather the *acquantainces* they'd *like-to-get-to-know-better* but would rather not admit to "stalking" by adding them on another network again.