(Hold on, I've gotta go hyperventilate.)
Don't worry. I'm here to personally assure you that Facebook has tons of already built in features that will keep your kid totally invisible from the outside world. If your kid is under 13, he/she is totally safe.
Another helpful article: http://blog.coherentia.com/index.php/2010/03/protecting-your-child-on-facebook-creating-the-account/
Your step child's account to Facebook has an email address and a password. In my opinion, your husband should know both of them and if he chooses to share that information with you, great. After all, this is a child (biologically yours or not) living under your roof, using your computer and your Internet connection. And you love this child. You want to protect them and make sure they're safe.
You and your husband have every right to see what they're doing online. At any time.
I get that it seems like you're invading their privacy, but really, it's time to get over that. Do you really think my mom respected my journal's DO NOT READ THIS OR I'LL FIND YOU AND RIP YOUR HAIR OUT sticker? No. How else do you think they found out about that Perkins waiter? I'm not saying you need to get in your step kid's account and snoop every day - but it is important to know you have access if you suspect something amiss. This is especially true for a younger child. Once they're 15 or 16, maybe reconsider if they've shown that they can handle an account sensibly.
I will say this. As a step parent, you might consider letting the biological father be the one to bring up issues if something inappropriate is discovered. Yes, you are a parental figure, but they only have one father and one true mother. Should you find something suspicious, I'd suggest telling it to your husband and letting him lead the charge in doing something about it. That doesn't mean you have to sit in the corner with your hands tied and tape across your mouth. You have input and you should share it. Just let Dad talk first and establish his role as a parent.
If your step kid has a FB account, chances are, you are one of the 27 people they actually know. They're probably going to "friend" you. Before you accept the request, remember to edit out photos of you dancing on table tops from your bachelorette party. Or at the very least double check your settings to make certain your child can't see those slightly less than "Step Mom of the Year" photos of you.
If you're like me and have a tendency to pen status updates about how excited you are to be reading that really smutty "Fifty Shades of Grey" trilogy, re-think letting your step child have access to your status updates as well. It may seem strange to them that all they can see is your name, your marital status and your graduation information, but tough. You are an adult and they are not. There are parts of your life that are not for their viewing or knowing. Just because your step child's life on FB is beginning, doesn't mean yours has to end. Be smart about how much you share.
What tips do you have for other parents whose step children have been given Facebook accounts?