There have been several proposals that I think would be attractive to people in my social network and I'd like to promote them.
I'm confident if they got a chance to use some of the sites that are still in the proposal phase they would become regular visitors. However, most of them have no experience with the Stack Exchange platform or understand the value it adds over any given message forum site. I just don't see them registering and following a proposal on Area51. Their interest is in the proposed topic, not the whole meta-Stack Exchange community.
Perhaps it would be beneficial to lower the bar for a site making it to the beta phase in terms of commitments? Even if it means that sites are more likely to emerge and die, what's the real harm? Essentially, it would be much easier to draw traffic to these sites if people could actually see them and the value of the platform than simply vote on a proposal.