I don't know about your country, but in mine (The Netherlands), it is a completely and utterly valid way to enter a contract.
Actually, you are free to enter a contract in any way possible. It is vormvrij (translated: form-free). Excluded is the purchase of a house, as far as I know. But for the rest, you are free to come to an agreement via WhatsApp, Facebook, email, or a scrawl on a piece of paper.
Actually, you are free to enter a contract in any way possible. It is vormvrij (translated: form-free). Excluded is the purchase of a house, as far as I know. But for the rest, you are free to come to an agreement via WhatsApp, Facebook, email, or a scrawl on a piece of paper.