Maybe. Or maybe not. Human psychology and motivational psychology are tricky things. Would it be shared less if people thought it had an agenda to get them to join some site? Do people join new social media sites the first time someone shares something cool and asks them to join (or of they do, do they stick around)?

Long term grown, IMO, will take people seeing great content over and over again to the point where they will want to join and use the site, regardless of if someone holds their hand through the signup process. That's just my opinion, though.