525 million registered Google Plus user profiles are out there in public.
You can see them all for yourself. (Data protection concern)

How can you see all the profiles? What can you use these many profiles for? What do these numbers indicate? Does Google need to take any corrective measures?

How can you see all the profiles?
Quick answer: Visit this link http://www.gstatic.com/s2/sitemaps/profiles-sitemap.xml
Detailed answer:
This week, I stumbled upon the robots.txt file of Google while doing some research for Search Engine Optimization.
While looking around the contents of the file, I noticed http://www.gstatic.com/s2/sitemaps/profiles-sitemap.xml towards the end of the file. Seems interesting! When I browsed this sitemap, I could find that it is linking to 50,000 more files which happen to contain all the Google Plus profiles and the (older) Google profiles.

Currently, each of those linked files contains links to approximately 11,500 more profiles. Out of them, around 10,500 are Google Plus profile.

Now, here is some simple calculation:
50,000 links x 10,500 profiles = 525 million Google Plus accounts.
Around 50 million of the (older) Google profiles have seemingly not migrated to Google Plus (or majority of them might be inactive accounts).

What can you use these many profiles for?

• A person might download and analyze this huge amount of data and use it for various marketing purposes.

• An evil user might be able to download and misuse this data which might include over 250 million authentic pictures of people (assuming every second Google+ profile has a real picture).

What do these numbers indicate?
Facts and my guesstimations:

• As Google+ had reached 500 million users around December 6, 2012. Probably Google+ is adding 12-18 million users in a month.

• The number of Google+ users is lagging way behind the number of android users and the gap is increasing day-by-day with around 1.5 million android devices being activated daily.

• It seems that the conversion of old Google Profiles to Google+ has stagnated. Probably a majority of the remaining ones (50 million) are the inactive accounts.

• Assuming two-third of the desktop/laptop GMail users must have tried Google+ for at least once (and usually people don’t delete their linked Google+ profiles, even if they don’t use it), there would be over 850 million GMail users all over the world out of which 75 million might be inactive accounts.

Does Google need to take any corrective measures?
Yes.
Privacy of user’s data is an important concern for every site. This issue was first noted around 2008 in one of the zdnet blogs. I believe Google can follow the way Facebook tries to handle this situation. Facebook provides its sitemap at http://www.facebook.com/sitemap.php which allows only authorized search engines and crawlers to index the user’s data. If you try to access that, you will only see a “Page Not Found” message. This provides the best balance between searchability and preventing stealing of all public data at once.

Disclaimer: The numbers in this post are based on facts and it has also been specified wherever estimates have been made.