[mathhead]

Clearly headlines play the biggest role here.

[lowglow]

I agree. Headlines are actually the grabber in any content you post. What helps people to join your community is engaging and pertinent content -- this is difficult. If you just want randos visiting your site to pump up your metrics, you can rely on cheap tricks.

[hayksaakian]

The issue arises when headlines' influence is in an order of magnitude.

At that point, even if you convert 3x less to your community, it's still worth it.

3000 visitors ---- 300 members (10%) 100 visitors ---- 20 members (20%)

I think that's what keeps aggregators afloat.