This doesn't mean Google+ will be a failure though, because unlike what he asserts, Google+ does offer one thing they don't: A tight integration with the Google services. Each of the various services used to have their own ways of sharing, now (or soon) they will have the same interface, namely Google+.
People may still use Facebook to keep track of old acquaintances and twitter to follow celebrities, but use Google+ to share photos from Picasa and their Android phone with their family, share documents from Google Docs with their colleagues, and Hangout with their friends. Which also explain why Google+ put Circles first, the various services doesn't overlap that much.
Joshua Gans will most likely be an active user of Google+ because of one these services, namely Google Search. Google Search is being tied to Google Profiles through the rel=author tag. This will put his name and picture next to his writings. The user clicking on them will go to his profile page. If he also share links to his writings on Google+, these will be shown alongside his profile. That will be hard to resist for any blogger.
http://www.google.com/support/webmasters/bin/answer.py?answe...
TL;DR: Google+ mainly makes Google better. It has overlap in functionality with both Facebook and Twitter, and make take marketshare from those, but that is not the sole success criteria.