It's a lot easier to find other people on Google+ based on interest. When I started using it, very few of the people I knew already were on there. I was able to search for other people interested in videogames, anime, programming, etc, and rather quickly I was part of a fairly large group of people reading and commenting on each others posts.

Discovery in Facebook is centered around groups and pages, which have a higher UX cost compared with posting a status update. On Google+, you can be found by anyone right away. And people use the sites differently because of this.

People make public posts on the assumption that what they write will be found by people they don't know yet who are interested in what they write. Facebook status updates on the other hand are usually always intended for people that the poster already knows, whether posted publicly or privately.

In the time I've been using Facebook, I can't recall once being friended (or now subscribed to) based on interests (as opposed to some existing connection). On Google+ however, I receive a new asymmetrical follow almost daily - and I'm someone with a rather low posting frequency.

This is what makes Facebook and G+ different.

[1] http://www.ftc.gov/opa/2011/11/privacysettlement.shtm