Just a quick heads-up for those who don't know yet: there is closed group "Astronomers" on facebook. It is by invitation only, ie some existing member must add any new member. You must have an fb account to be added.
The group has about 4,000 members, most all professional astronomers from around the world.
There is some interesting discussion going on in that group and some useful info being passed around.
The closed group feature seems to work well for some combination of intensity of topic focus and size - not too small, not too big.
'fraid I wouldn't tell how to monetize the feature though.
The thread on murdered astronomers was particularly gruesome and interesting.
By the way, your pretty picture on every article seems to be 404 - not found.
yeah, the move is having some shakedown issues
we'll get there eventually
Useful info, eh? Would you say it's useful enough to justify joining Facebook?
Hmm. So the group has a bit of a flavour of an active moderated usenet group. The discussion draws in senior bods who wouldn't be caught dead commenting publicly in other social media, and is quite frank, since readership is pros only.
Little bit of trolling, handled ok so far, might crack badly if someone sends out too many "adds".
In some sense the closed group is achieving what Google+ circles are evidently intended to do, whereas my sense is the circles are failing because of the way they are co-added.
I don't know if it is worth joining fb just to get onto Astronomers, I think the major discussions overflow into other social media; eg AstroBetter is actively working to manage discussion flow like that.
More if you are on fb for non-professional reasons then there is now an interesting professional hook.