At least in Canada this is a legally binding contract (Source)

A Canadian court has decided that a thumbs-up emoji was used to validly accept and create a legally binding contract

The court also found that a thumbs-up emoji, whilst non-traditional, was a valid electronic signature under Canadian legislation and fulfilled the two purposes of signatures, namely, to identify the person signing and to express acceptance of a contract.

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@trixwit No? In your example both parties have a reasonable expectation that a serious agreement is intended. Elon's post on Twitter would be interpreted by most people as a joke.