Facebook is a good start, but you might find iAds a little pricey when just starting since I assume you don't have all your metrics down yet, like LTV, ARPU, etc. These are very important since you'll know what you can afford to spend to acquire each user.

As next steps there are many networks I could suggest, depending on what your app is, including InMobi, Millennial, Ad Colony, Aarki, Greystripe, etc.