I personally prefer G+ over FB but it doesn't have as many users. I suspect that will change the more FB pursues it's commercial revenue streams. I think people will be bailing once they get a full does of intrusive advertising every time the log on.
I agree personally - it is easy to get frustrated over Facebook's attempts to nickel and dime you. The "boost post" button irritates me. However, so far the service itself is free.
The problem with G+ is that a lot of people aren't computer literate enough to feel the need to have a Google account. G+ is to Facebook as Firefox is to Internet Explorer. I do like the suggestion, though, and may branch out to G+ as well.
A colleague of mine suggested creating a Wiki page for our observatory. While versatile, I don't feel that it will find the audience that the social media websites do.