I don't advise using FB for casting calls. The potential for having a flash mob or worse show up is too great. If you're looking for a free production source online, try mandy.com they are a primary source for advertising casting calls and more. Just so you know, you get one free announcement a month and there's a fee for any more than that.
If you want to know how much to pay actors, go to the SAG AFTRA website (sagaftra.org) and check their current wage scale listings. If you're planning on hiring union actors and talent, you'll need to set up an agreement with the unions for them to authorize the work. The rules are binding once you agree so you better have your funding, working conditions and much more squared away or your production will be in for a world of hurt. If you either can't affort union talent or don't want to hassle with a union agreement, you can still hire non-union talent. Base your pay on a percentage of the union scale (but don't get too cheap or no one with skill will want to work for you!) You will still have to comply with state and federal hiring regulations and still have to have a contract or agreement made up stipulating plainly what is and is not expected during their preparation, performance and any needed post-production performances. If for whatever reason there needs to be any level of undress if not nudity, you'll need a separate clause breaking down in detail what will be required and what extra compensation will be offered if any.
Truthfully, you sound like you need to do quite a bit more research before you make a move in this direction. It wouldn't hurt for you to partner up with a producer who has a grip on this part of the business to help you get the ball rolling smoothly.