I find a very easy way to get your first X users if you don't care much whether they are in your core target audience (but they should be in same ballpark) is to do a giveaway for blogs with a decent-sized subscriber base. Give away $20 Amazon gift card but to enter you must sign up for an account, and do something. $20 for 50-100 users is in the realm of possibility.

The great thing about this is that it does 2 things: gives you those first X users for social proof and it gives you a head start in SEO by building "organic" links (though one might argue they're paid technically)