We asked 1000 Subaru drivers what they thought of cars...
When you ask people who went out of their way to choose a product what they think of it, you're far more likely to get a positive response.
The Google+ users might be randomly selected for this survey, but the entire pool of G+ users is more likely to be self-selected than Facebook's userbase is.
Put it this way:
Take a sample of 1,000 active G+ users, completely at random, call this G.
Take a sample of 1,000 active Facebook users, completely at random, call it F.
1. What percentage of G could be classed as employees of Google, or evangelists for the company's services (paid or otherwise)?
2. What percentage of F could be similarly classed as Facebook employees, or evangelists for that company's services?
I have never encountered a "Facebook fanboi" on the internet or in real life. Most attitudes to the service (mine included) are that it's like the phone company. Sure, it's annoying sometimes, but it's a means worth putting up with, when the end is being able to stay in touch with people you care about.
No offence to Subaru drivers in the headline, by the way, but they're a famous example of an enthusiastic and vocal self-selected grouping, and one that tends to overperform in ownership surveys for this reason.