Depends on the amount of funding and how much equity you're getting.

If they're well-funded (Series A or later) and you're getting negligible equity (hopefully not), then you should take the same salary as an established company like Facebook or Twitter would offer you. Probably a bit over $100k base for 3 years of experience.

Take that starting point and adjust it for the amount of equity they're offering and how much you think it's worth. Now your goal is to get them to pay you no less than you're worth. :)