Three months after the launch, Google+ no longer requires invitation and it's available to anyone who has a Google account. "For the past 12 weeks we've been in field trial, and during that time we've listened and learned a great deal. We're nowhere near done, but with the improvements we've made so far we're ready to move from field trial to beta," informs Google.
The service has improved a lot, more quickly than any other Google product ever released. After announcing the public data APIs, Google added support for Hangouts in the Android app and added the option to broadcast a Hangout, but only for a small number of users. "We're starting with a limited number of broadcasters, but any member of the Google+ community can tune in. In fact: we'll be hosting our very first On Air hangout with will.i.am on Wednesday night, September 21. For more information visit will.i.am's profile on Google+."
Hangout has many other features you can preview by clicking "Try Hangouts with extras": screensharing, sketchpad, Google Docs integration and named hangouts "for when you want to join or create a public hangout about a certain topic".
The search box is now more useful because you can use it to find posts from your friends and from other Google+ users. Google also lets you restrict your results to people and save your search.
Google doesn't disclose the number of Google+ users, but a Comscore report from August estimated that "in just one month, Google+ has captured 25 million visitors". It's likely that Google+ has a lot more users today and the number will grow, now that the service no longer requires invitation. Google+ is the first social service launched by Google that's actually successful.