Facebook billing does function differently than regular StreetText billing schedules. Facebook also operates on their own billing schedule, and StreetText has no insight on when certain accounts are billed. With that being said, here is a breakdown of the Facebook billing side so you can learn when to expect a charge from Facebook on your credit card. 

When you sign up with StreetText, you will choose your Facebook ad(s) budget that is billed separately to your StreetText subscription. This amount will vary from person to person, however Facebook will always bill every time the ad(s) budget hits the billing threshold of 50$, or at the end of the month - whichever comes first.
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If the 50$ threshold has not been reached at the end of the month, they will bill you for whatever is remaining.

See below for a breakdown:

If you are billed from Facebook multiple times a month, don't fret. This will stay consistent with your monthly budget that you set, and will never go over. 50$ is just the increments Facebooks requires for billing purposes.