Considering Google killed off Orkut at the end of last month, and still haven't killed blogspot/blogger i suspect Google+ will hang around for a while longer.
It's a lot more integrated into their other services than Orkut and blogger ever were as well.
I suspect Google+ will morph into a "Value added" social backend for some of their products, youtube, hangouts, gmail all have Google+ hooks. They are starting to use Google+ to rank your searches, I see my friends posts in my search results often now. They could use this to add personalization to Google Now.
In a couple of years Google+ may be closer to Discus's "embed everyone" model than Facebook's silo. They might end up using Google+ to integrate the data across their products, which were (and still are) siloed until recently and just remove the Google+ homepage.