Which is correct and why? (Pienso en usted)
(Pienso de usted) for I think about you

See a translation

For "I think about you" is "pienso en usted" but as the comments above said "pienso en ti" sounds more natural in an informal way.

"Pienso en ti/usted" You probably think about what that person is doing, or you remember some moments that you both have shared (I mean in a romantic way)

"Pienso de usted" is correct in other context. "Pensar de" is used to ask for opinions about something (in this case about someone)

*¿Quiere saber lo que pienso de usted? /Do you want to know what I think of you?

This question is also formal because the person who asks is using "usted"

Another example:
¿Qué piensas de Ana? What do you think of Ana?