1. Your former colleague
Of course, you do not need write greeting card to all the former colleagues, but if you have a former colleague who is special close contact frequent, send a greeting card is a very warm thing. So in the future if you have something ask for help, it will not seem too unexpected.
Hope you have a pleasant holiday! I hope you are happy in the [company name], [department name] everything is good. I am very fond of [work together / go to the cafe / together in the company activities ] time. If you go to my company / want to have a cup of coffee together, please contact me.