Call me hopeless newbie, but I really haven't thought about people who may not sign up right on their first visit, but may do so later - and a way to "track" them.

I think the social buttons you've mentioned are good for this too - if people tweet/share and get feedback, they may be more likely to come back to the site.

I'm not happy with Addthis, the social solution I'm using. I chose them for analytics, but I'm starting to feel that sticking with the default buttons might generate more engagement.

I really have to put more thought & effort on social. Thanks for reminding me of that!