Google+ offers the following core functions.
Circles let's you organize your contacts into distinct and walled-off groups. Stream updates you on activities from your circles. Sparks serves you up content based on your interests. Hangouts is a group video chat feature. Huddle and Instant Upload works with mobile phones.
Circles and Stream are what people tout as Facebook or Twitter killers. But that's likely not true because Google+ doesn't quite do the same things.
Facebook is the persona and profile you have online. Twitter is what you want to say to the world and how you hear what the world is talking about.
I don't imagine a scenario where #RIPCelebrity hash tags move from Twitter to Google+. Or Lady Gaga transferring her 11 million followers to Google+. Or Ron Paul internet supporters switching the Ron Paul 2012 Facebook page to Google+. Or college students making friends with fellow Star Wars fans by joining their university's Star Wars Facebook fan page.
The point is that Google+ is Google+, not Facebook or Twitter.
There will be overlaps, of course, but most users will likely use Google+ for what it is, which is a tool to share content on Google platforms seamlessly and easily create segregated groups of contacts.
The single biggest factor to whether or not it'll succeed is how attractive it is to users, not how it stacks up to Facebook or Twitter.
Indeed, the social networking space is likely big enough for all three products to survive and thrive.