Ahh, Google Plus. Currently populated mostly by the earliest adopters, the people that Google prioritized when they handed out invites: tech journalists, then social media gurus, then (by crowdsourcing the invite system) anyone else who was the slightest bit interested in the field. With an initial userbase like that, it's not surprising that the internet was abuzz with detailed analysis of the Google Plus system and features from the very beginning. What is surprising, though, is that most people didn't really seem interested in Google+ itself. Rather, they just wanted to argue back and forth about whether it was a "Facebook killer". Its virtues, its flaws, its unique features - none of them escaped being compared to other social media offerings.
Of course, it's a widespread practice in tech reporting to declare that one company or another has "killed" all competitors and crowned itself absolutely supreme. The laptop has been declared dead, the netbook has been declared dead, and just about everything else from fax machines to the Web have had their obituaries prematurely posted over the years. Yet even the humble telegraph remains in use in this era of widespread wireless, and portable cassette players were in production as late as last year despite the existence of the MP3 player. Old tech may no longer overwhelmingly dominate the way it did when it was new, but it still commands a respectable marketshare, and even ancient tech still thrives in its various niches.
Similarly, the chances of either Google+ or Facebook dying off anytime soon are virtually zero. After all, even Orkut and Buzz still matter depending on what you're doing; Orkut is notoriously popular in Brazil, while Buzz still features prominently on the standard collection of social media buttons that just about every site on the web has nowadays. Google Wave would probably still have a few percent worth of marketshare if Google hadn't gone out of its way to kill it off.
With that in mind, it's nothing short of sensationalist to say that either Facebook or Google+ will be in danger anytime soon. They're far more likely to share the market, with each holding 50-80% marketshare (remember, it's quite possible for a single user to use more than one site, and many already do!) - just like every other social media site does. It's not like Facebook is stifling the likes of Twitter or LinkedIn, and while Google+ is admittedly much more of a direct competitor to Facebook, I don't think they're so similar as to be mutually exclusive.
What will this lead to? Probably, a more diverse social media ecosystem. Just look at how many features Facebook announced in the days following the Google+ launch, many of them in direct response to the new features that Plus brought to the table. It seems clear that the intense competition about to begin will bring plenty of rapid progress to the social universe, and the group that's likely to benefit most from this fight is the users. We might just be looking at a "Golden Age" of social media innovation, and the rapid change that such a battle will bring may very well be enough to convince users to use BOTH sites for various purposes.
In the unlikely event that either of the two sites "wins" and manages to completely crush the other, the winner will have changed and evolved so much from its current state as to be almost unrecognizable - so regardless of what you think, any predictions based on the sites' current states aren't likely to be reliable. Especially in the case of Google+, which is still in development, so it's a still little early for self-proclaimed prophets to announce their predictions. And with any luck, an overwhelming victory won't happen like that anytime soon, or else the victor will take their domination as an excuse to let their site stagnate.