"Plus has 29 million unique monthly users on its website and 41 million on smartphones, with some users overlapping, compared with Facebook’s 128 million users on its website and 108 million on phones, according to Nielsen."
Is that supposed to be a bad thing? If you have ~1/4 to ~1/2 as many users as Facebook, are you not a very successful social network? If Plus was an independent startup people would be crowning it the next Facebook and saying that Facebook better watch its back. With all numbers in this article the author seems to say that if you don't have as many users as Facebook, you're nothing.
Plus is a Top-5 social network that also offers a convenient solution to Google's many-account problem. Sounds like it's doing just fine.