To get people to move from one social system to another you need:
Something dramatically better to move to. AOL beat Prodigy because it had a better UI. Compuserve beat AOL because it was faster and had more intelligent conversation. Usenet beat Compuserve because it had more choice. The Internet beat Usenet because it had even more choice, but also was better integrated into other things, was easier to develop for, and eventually had far better spam and noise control. Etc etc.
You’ve gotta get influencers to move AND stay in the new place. Google+ got some, but most of the influencers are still available in Twitter and Facebook. I have never seen an online community get popular with non-influential users first and then the influencers follow. It ALWAYS happens the other way. Which is why I watch what influencers are doing so much. (Modern equivilent, what are people with Klout scores of more than 50 doing).
Provide infrastructure for future platform that others can’t react to. The Internet eventually beat AOL, despite AOL making billions, because communities on the Internet supported the web browser, which got very popular starting in 1994 and more popular every year since. AOL could never react to this new platform, so users left, eventually. Imagine that Google+ works with those new Google Glasses better than Facebook does and that the Google Glasses get very popular. Will Facebook be able to react? (Today’s shifts are happening because of mobile — look at why Twitter’s tweet volume doubled last year).