Comparative adjectives: using than
We use than when we mention the second person or thing in the comparison. If the second person mentioned takes the form of a personal pronoun, we normally use the object form of the pronoun (me, you, him, her, us, them):
Could you carry this? You’re stronger than me.
Not: You’re stronger than I.
Why did you choose Robert? Marie is more experienced than him.
In more formal situations, instead of than + object pronoun, we can use than + subject pronoun + be:
You managed to answer the ten questions correctly? Well, you’re definitely cleverer than I am!
I preferred Henrietta to Dennis. She was always more sociable than he was.
The comparative form is used for comparing two people or things:
He is taller than me.
Speech you don't prefer cannot just blithely be disregarded as informal though. There is nothing that officially makes "je ne sais quoi" formal but "yanahmean?" informal - even in that regard, it comes down to personal preference.
Formal tends to be more correct, informal less so. But there is no strict line or percentage of 'purity' to delineate the two. IMO.
This is a debate that goes on all over Duolingo. The full sentence is "She is thinner than I am." It is true the the incorrect usage of "me" has infiltrated the language, and it can be found everywhere, but that doesn't make it right. A lot of people say "Him and me went to the disco," but just because a lot of people say that doesn't make it correct.