According to comScore Inc., the number of Facebook users in the United States is up approximately 5 percent year-over-year. This is a substantial slowing of growth relative to prior periods.
Facebook has 850,000,000 users. Google + apparently has 250,000,000, and is adding about 500,000 per day. So if google plus kepp growing at their current rate they will need to add 600 million users, that means about 4 years to catch facebook.
The big issue for G+ is currently the average users spends 3.5 minutes per month on G+, whereas apparently the average is 7.5 hours per month - that is a massive gap.