See this good explanation from the Oxford Dictionaries: (informal) A brief but intense infatuation for someone, especially someone unattainable. Note all the qualifying adjectives. Infatuation is not really "falling in love", is it?

"I have a crush on someone" is another that means that you like that guy or girl or fancy the person in a way you would want to develop a relationship with them such as dating.

Yes, having a crush on someone is often used, but mostly by younger people in high school or pre-teen years. For example, many years ago (or it could have been minutes also), I had a crush on a beautiful Persian girl with deep confident eyes and a warm smile that projected sincere friendship. Her voice was cute and full of coquettish intelligence. I learned a crush can transcend; distance, time and realities.

She has a crush on you = Ela está a fim de você. And yes, it is commonly used.

I think you mean to have a crush on someone, it means to fall in love with that person.