She should make use of websites used by a large number of people and from where information can be shared quickly.
Step-by-step explanation:
Since Selena Gomez has to make the people aware about a particular contest with which these people might be able to earn some lucky tickets giving them free entry to a concert, the site or the platform that should be used to make people aware about this contest or give information about it should be the one which is used by a large number of people and from where it can be shared quickly from one person to another.
For this purpose, Selena should make use of such websites for giving the information.