You use expressions such as `I bet,' `I'll bet,' and `you can bet' to indicate that you are sure something is true. [informal] I bet you were good at games when you were at school. I'll bet they'll taste out of this world. See full dictionary entry for bet.
The term bet can be used in a few different ways on social media but generally means agreed or okay. For instance, someone might say, Want to grab lunch later? and the other person responds, Bet . It can also be used as a shorthand to mean you can count on it or trust me.