Anyone remember the headlines several years ago when "Google 'scans' your emails to serve targetted ads'? When was the last time you heard someone complain about that anymore? No one does, because it doesn't matter.
Why should me sharing a link via a PM and my action be aggregated into an anonymous number worry me? How is the argument different from Google 'scanning your emails'?
Social networks nowadays seem to be competing on ways of justifying bigger numbers on the counters. It makes sense, the bigger the number the bigger the social proof which will attract webmasters. Scanning PM's for shares isn't something I might of thought they would do, but they do and it makes sense. The whole privacy debate surrounding this particular case is a complete non-issue for me though and will probably fade into insignificance just like Google scanning your emails.
An action from a big company that shifts the boundaries of acceptability in privacy would be a justified concern, but in this instance no land here in the privacy war has been lost. Nothings really happened.