There are many differences between the two, but this one I haven't heard discussed before. 

Facebook came from Harvard, and grew among college students on other campuses, way before anyone in Silicon Valley used it. Everyone from the tech industry feels like a late-comer, not an early-adopter. Something that makes all of them feel a lack of ownership. 

Google-Plus is starting from Silicon Valley, the usual insiders have the most followers long before any of the usual Facebook users are allowed in. Probably before most of them are even aware there is such a thing as Google-Plus. 

Things like this tend to matter a lot more than you think they should.