As mentioned in another answer, they may not have made the decision early on Friday. In my experience, issues of various sorts can delay decisions for several days or even weeks, although things are generally faster now than they once were.
Contacting those you interviewed with can be a double-edged sword: If you don't follow up at all and the process takes longer than planned, the hiring manager may think you are not interested or have found another job and drop you from consideration. Alternatively, contacting the hiring manager may annoy her/him and lead the decision away from hiring you. Unfortunately, there's no sure way to know what reaction you will get. In fact, the reaction may depend on the the hiring manager's mood when contacted.
That said, my suggestion is to ask them. Ideally, at the end of the interview you would establish some sort of plan for how future contact should go. It seems you did that somewhat, but not in detail. In the future, it might be good to say something like "Since you plan to make a decision by Friday would it be okay for me to contact you on Wednesday of next week?" This gives them some time and (hopefully) keeps you from seeming over anxious and annoying. If their answer is "Don't call us, we'll call you." (or similar), that definitely tells you that they don't want you to bother them ... and may indicate you aren't their first (or maybe even second) choice, but that they haven't ruled you out entirely.
In this case (assuming no plan was set up at the interview), I recommend waiting a few days after they said they made their decision and then sending an email to inquire. An email has the advantage that it usually won't intrude into their day as much as a telephone call and allows them to answer on their own schedule. In it, you can ask about how the process is going and how frequently to make future contact to keep up on the process, as well as whether they've made a decision and if it is you, of course.
One note: Sometimes you'll be told that no decision has been made or a delay has occurred when that's not really what's going on. This sort of answer is sometimes given when your are considered a viable candidate but not the first choice. This way they hope to avoid telling you they prefer someone else, but will take you if that someone else doesn't take the job.