Some 7 years ago, following successes in physics, ChemWeb.com launched the Chemistry Preprint Server (CPS), and Warr evaluated it in a JCIM article three years later. She wrote about 'lessons learned', but the only one seemed to have been that chemistry was not ready for it, as the project shutdown in 2004. The archives are still available, fortunately, and you may find it amusing to look up my or some other submission.
Now, Nascent wrote that Nature is setting up Nature Precedings, which was earlier noted by Pedro. The official announcement was published as an editorial in Nature. This being a Nature initiative, and not focused on just chemistry, I am sure it will do better than CPS. BTW, media coverage is tracked in a social way.
I might request an test account; I do have an old half-finished manuscript that I never got around to finishing. While still relevant, it could use some community input; this preprint server would be the perfect tool. That's how my first manuscript ended up on CPS too :)