Some estimate that Google+ will reach approx 20 Million Google+ Users by the Weekend. This would translate to a faster growth rate or user adoption/signup rate than that of facebook.

At 20million it would take say 2-3 months to reach the levels of facebook, and being tied to the rest of the Google platform would make use and adoption that much quicker and seamless.

The challenge for Google is to get those that are on facebook and not on any of the Google platform products.