Your content may be shared beyond the bounds which you originally intended if your post is reshared or if someone is mentioned on your post. For instance, if someone is mentioned in a comment, they'll be able to see the entire post even if the post wasn't originally shared with them.So if you mention someone else in the content of the post, and happen to prepend that third person's name with a '+' or '@', then that person can now read your message, which defeats the purpose of a private, direct message.
That said, Google+ is still pretty cool, and one can argue that because there is this limitation of no direct messaging, Google+ is cleaner and leaner, akin to Twitter's self-imposed 140 character limit (which I, and Farhad Manjoo, think has outlived its usefulness, especially for Twitter conversations).
Before Google Plus came out, Google introduced +1, which is their version of the Facebook 'Like' button. In addition to Google's how to add +1 to your blog, for those on blogger/blogspot, here's what I did:
As stated in the Google document above, you need to copy and paste one of the following 3 possible pairs of XML elements into your blog template:
- <script> and <g:plusone> elements
- <script> and <div> elements
- In Blogger's tabs console, click on 'Design'
- Click on 'Edit HTML', and in the section 'Edit Template', click on 'Expand Widget Templates'
- Paste anywhere inside. I recommend that you provision a section inside the template so you can keep track of all the changes you've made to the default.