[colonel_panic]

I don't think the exclusivity was a long-term strategy, but rather a natural progression from its beginning as a site for Harvard undergrads. It was really useful at providing specific needs for college students at the start. It was easy to find who was in your classes so you could ask for homework help or invite people to work on a group project back when everybody listed their classes through Facebook's central app. But after it opened up to everybody and a whole bunch of apps appeared, there were multiple competing class-listing apps with no clear winner and you needed to use all of them if you wanted to see everybody in the class. It became less convenient and most students no longer bothered to list their classes at all.

Once Facebook understood the appeal the site had to the general population, they realized that it would be better for them to create an ecosystem where network effects were optimized for the decentralized user base as a whole rather than the centralized subnetworks that made the site take off in the first place. At each step, they focused on what worked for the users at the time.

Google+, on the other hand, really did deliberately start out as an exclusive network, largely for people in the tech world, and it hoped to quickly transform into something with universal appeal. You can draw your own conclusions about how well that worked.