In order for a contract, or anything else, to be implied (literally, "something suggested but not stated directly"), an action must be undertaken or a statement made that implies it. If I order at a restaurant, I imply that I am going to pay. If I'm in an 80's movie, ask if you want to have a good time and you give me some sum of money, it is implied that we will be having sex. Such 'contracts,' while not the most enforceable thing in the world, and subject to complications whenever the implication was not properly inferred (lost in translation), are as valid (assuming both parties understand the communcation) as if the contract had been stated outright, as the same agreement was communicated and mutually understood.
However, in the example of the social contract, and many others like it, a case is made of an implication where nothing has been said or done to imply agreement-- *inaction,* rather than an action, is taken as implication; e.g., by *not* leaving a "country," you imply agreement to abide by its laws. This is equivalent to having my car stopped at a light, when a drifter comes up, washes my windows, then mugs me, and saying that this is all perfectly legitimate as my non-moving of the car "implied" my consent, or that it is okay to rape someone in a coma and leave some benjamins on the nightstand. By the definition of the word imply, inaction cannot imply.