You're absolutely right, and google+ does give you the opportunity to do that. My circles are created based on interests. I have one for friends interested in politics, one for people who share my taste in music, etc etc. I only enjoy using facebook to share information with others and have them share information with me. If I can redirect my link to a specific set with out having to "spam" the rest of my friends with 20 links about a topic I know they're uninterested in, I would much rather do that. Alternatively, I tire of acquaintance B shooting out status updates about fly fishing when I have the opposite of interest in that. I hope they place me in an appropriate circle and leave those updates for the fly fishing one.

Google+ does understand its about sharing information, but just throwing information out into the internet abyss is pointless if it's not relevant to the audience.