There are several proposal-specific discussions on advertisement, but those often are very specific to the proposal. What I'd like to make available is a list of generic ways one can advertise a (relatively) new proposal---one that has not reached the 50% threshold to be automatically advertised.
Is there any other medium by which to advertise other than social networks (eg Facebook, Twitter), or any appropriate way to advertise on different SE sites (via tag wikis, etc)?
Unfortunately, we do not allow solicitation of users for proposals in meta support or on the Q&A sites. With so many proposals vying for attention, allowing proposal authors to solicit users for support in posts, tags, and comments became unworkable pretty fast. Such activity will be 'flagged' and removed.
I can appreciate the need to find a community for your proposal, but when you submit a proposal for a site, it is stated that you need to have access to an avid community to build it. You were asked to provide a description and links to the site or organization which is (presumably) heading up this effort.
Without that level of support, your proposal will likely be closed.
Some of our larger ("graduated") communities have a meta post tagged community-ads which may or may not welcome ads for proposals — do your research first — but the audience for those ads is small, and we cannot allow proposals to linger along with only a small accretion of users over time without a larger base of support.
It takes a lot of users to build a successful proposal.
Promotion should be done predominantly outside of the Stack Exchange Network.
