Try ALL the solutions mentioned by you or the others, and see where from you get more visitors and members? Also, watch the analytics (google analytics from blogs/sites or the stats offered by some social networks) and see what countries they come from. Use affiliate links to have more infos and see also some conversions...
Twitter is good and we prepare some twitter promo services for CamGirls and Studios through WeTwit4U:
WeTwit4U | CamMillionaires – from 5-6 figures/year camgirls to more than 7 figures!
We're also working on more solutions for promoting individual models or cammodels from studios...we'll add more info these days here:
CamMillionaires.com - Services for Elite CamModels!
Good luck!