As a user, I don't think the sharing aspect is all too important. I get links shared with me in many other ways, then I open them. If something looks interesting, then I add it to Instapaper.
In terms of Instapaper's sharing functionality the bit I really like is it's ability to produce an Instapaper formatted stream of articles linked to by people you follow on Twitter.
Sharing out I'm not so bothered about but a nicer way to consume links shared with you is cool.
Sharing out I'm not so bothered about but a nicer way to consume links shared with you is cool.