Some have speculated how significant this news is that Google+ is 4th, with reasons pointing towards that if users want to use YouTube, Hangouts or other Google services they need to have a Google+ account. While this is true, you do not need the Google+ app installed to use the YouTube app or even chat with friends using the Hangouts app. This reports demonstrates that users are actively using Google+, viewing their news stream and posting status updates.
Interestingly, there is no mention to the Hangouts app. It appears many users are using Google+, but it looks like Hangouts adoption may be slow in favour of Facebook Messenger, WhatsApp and even Skype. However, the report is only covering usage within the 2nd Quarter of 2013 (April - June). Hangouts was officially launched in May during Google I/O, so possibly we could see Hangouts appear in the list at the end of the year.