This is a tug of war to keep the people hooked to the sites.
Hope you remember the coming of Orkut. All were talking about it, but with the span of time when Facebook arrived all forgot Orkut. the reason was that these social sites are constantly invaded by the class of people who are having accounts for fun, just wasting time.
This will happen on Google+ also but now we have to wait and watch as to what parameters will Google apply after having bitter experience from Orkut.
The innovative team of Google will do something unique this time to lead the race of more hits per second.