If you are far away and not able to attend the birthday party and send a gift, then sending birthday greetings on a cute card is really a good idea. It will compensate your absence to some extent. They will feel about your concern and that will make them feel special.
While writing birthday wishes, keep in mind the age and personality of the person. For the kid you can greet and wish him or her for successful future. You can use some funny things in writing a wish for your friends. Some serious stuff needed for the old and mature people.