By now, most Gmail users have received their invitation to Google Buzz, the most talked-about social media software launch of the year. And yet the jury is still out. Mashable’s poll sees 39% of its very geeky readership as either anti-Buzz or on the fence. Not a good sign.

One of the problems is that Google Buzz doesn’t allow you to connect with Facebook’s 400 million users, and it does short shrift to Gmail itself. As John Battelle points out: “Buzz does not let you publish out from Gmail to Twitter or Facebook. So for this to compete, you have to build yet another network of followers/friends – and do it through Google services.”

Not surprisingly, Microsoft and Yahoo are chiding well, it’s about time.

Microsoft pointed out hours after the Buzz launch the big elephant in the room – do we really need another social network? “What they want,” Microsoft contends, “is the convenience of aggregation. We’ve done that. Hotmail customers have benefited from Microsoft working with Flickr, Facebook, Twitter and 75 other partners since 2008.” No surprise there; Microsoft is a Facebook investor.

But, with over 145 million Gmail users, Google does have a massive installed base with which to challenge Facebook and other rival social networks.

On the enterprise front, the future is more intriguing. Google could use Buzz to compete against Microsoft and Cisco by building it up as a unified corporate communications platform of its own. It’s already begun staking out this market, starting with an enhanced biz-friendly Google Voice and Google Wave offerings. To be sure, Google is keen to crack the enterprise market, primarily by building a suite of heavy-duty corporate applications that help companies connect their disparate units. Could Google Buzz be the Trojan horse that ties it all together?

Ultimately, how Google Buzz plays out will be largely determined by Gmail user take-up and by the level of support Google puts behind the relatively limited number of applications featured in Buzz 1.0; today it’s all about sharing multimedia, links and status updates via Flickr, Picassa, Twitter, YouTube and Google Reader. But what about voice, not to mention its Web site hosting offerings (Remember Googlepages?) that have left small business operators stranded in cyberspace?

As Paid Content’s Robert Andrews Tweeted this morning, summing up the frustrations of many early adopters: “Google must open ALL its services (not just three) to Apps users like me or Buzz will be inaccessible to tens of thousands. Grr.”