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There has been a frenzy of recent reports about the user base of Google’s (s goog) new social network, Google+, and how it has allegedly declined by a massive amount since it opened to the public several weeks ago. As it turns out, much of that is likely a normal cooling-off period after the initial explosion of interest in the former invite-only beta, and Google+ numbers are still substantially higher than they were before the beta wall came down. That said, however, the new social network has a substantial mountain to climb in terms of gaining a broad user base; it’s the third entrant in the race with Facebook and Twitter, and it’s still not clear what Google has to offer that is radically different from its competitors.

The initial report about the gigantic decline in Google+ user numbers came from Chitika, an ad-tracking service, which reported that the traffic to Google+ had dropped by more than 60 percent from the levels seen immediately after Google opened the network up to the public. This report was picked up by a number of blogs, which speculated about the reasons why Google’s offering might not be gaining widespread acceptance, but others noted that while Chitika’s figures (the methodology of which hasn’t been made clear) showed a dramatic falloff in usage, Google+ has still shown strong growth since it first launched in June. When measured from that initial launch point, the service’s user base appears to be up by almost 500 percent.

In fact, as we’ve reported before, Google+ hit the 25-million-user mark faster than almost any other social network in history, including Facebook and Twitter. While those numbers may pale next to Facebook’s 800 million users or even Twitter’s 200 million, there’s no question that Google is already a strong third in the category, thanks in part to the fact that Google+ is embedded in most of the company’s other services — along with the toolbar that constantly prompts users to check their accounts. And recent comments by director of product Brad Horowitz made it clear that Google’s new social network is going to become part of everything the company touches.

What is going to compel users to spend time on Google+?

We know why Google wants Google+ to succeed: because it needs to capture those social signals in order to enhance its search. But is there anything compelling enough to convince large numbers of people that they have to use it? I’m not so sure. In looking at my own usage of the network, and that of some close friends and members of my “social graph,” I can see a lot of dabbling in Google’s service — and the occasional flare-up around a specific post, usually a strong opinion or a photo — but not much sustained usage, apart from people like Robert Scoble (and even Scoble says he’s still using Twitter a lot). In a recent post, he listed a number of reasons why he prefers Google+, including:

The length of Google+ posts means “I can have a complete thought,” while Twitter is much shorter (obviously), and Facebook doesn’t really cater to longer posts. The way that Google’s service allows comments means “I can have a REAL conversation THAT STAYS BUNDLED TOGETHER here,” unlike Twitter, which is fragmented. The fact that Google+ is part of the world’s largest search engine means that “the search here is very good,” unlike both Twitter and Facebook. According to Scoble, photos and videos are “MUCH BETTER displayed” on Google+ than they are on Twitter, which has tried to upgrade its hosting of images.

But even Scoble’s list of benefits only highlights the challenges that Google+ has ahead of it, particularly since Facebook has launched some competitive features that duplicate some of the service’s features — including the use of “Circles” for specific user groups (Facebook added smart lists) and support for an “asymmetric” following model similar to that offered by Google+ and Twitter. So what are the things about Google+ that make it unique, or are going to make it so compelling that people will take time out of their day to engage there, instead of just using the networks they already belong to?

Is it longer posts and conversation, or photos, or search?

Is it about longer posts? Because Facebook now also offers support for longer posts, and since most people already have a well-established social graph on the giant network, those posts are likely to get far more interaction from people who really matter than on Google+ — where much of the response I get is from people I don’t even know. While valuable in many ways, that’s a different experience. So is Google+ about filtering via Circles then? Because Facebook’s smart lists actually do a pretty good job of that, at least in the short amount of time that I have used them (although I wish they were supported by Facebook’s new iPhone and iPad apps, (s aapl) which so far they are not).

So is Google’s big advantage photos and videos, as Scoble suggests? I’ve seen lots of activity around images and video on Google+, which makes it very easy to share and comment. But I’ve also seen plenty of activity around that kind of content — in some cases much more — on Facebook, which is also pretty good at displaying photos and video. And when it comes to pure information delivery, about news events or items of interest, I agree with Scoble that Twitter is almost unparalleled, which is why I don’t think Facebook’s launch of a “subscribe” feature is likely to harm the service much. In my usage at least, Google+ hasn’t proven to be much of a competitor in that department either.

So what is Google+ offering that users can’t get on Facebook or Twitter? I can’t put my finger on anything, with the exception of search. Although it didn’t launch with search initially, Google added it to Google+ before it opened to the public, and it is a great way to find content — something notably lacking at both Facebook and Twitter. The latter still has a pretty terrible search experience, and users can’t find older tweets or context without using third-party services such as Topsy, or find trends and other activity easily. This is something Google is (not surprisingly) pretty good at.

That said, however, is good search enough to build a compelling service that hundreds of millions of users will feel they need to engage with on a daily basis? I’m not convinced.