Is it considered a sin to leave pictures of myself with my abaya on my shoulder published on Facebook, where men and women can see them, knowing that I was unaware at the time of the impermissibility of that, and I am present in most of the pictures, with my shoulder appearing in some of them?

You must take the initiative to request the removal of your photos from the pages where they appear, because your face is visible in them according to those who consider a woman's face 'awrah (private/concealable), and this is the prevalent fatwa. As for the shoulder abaya, the purpose of hijab is the concealment of a woman, and both types of abaya (shoulder or head) achieve this purpose. The preference between them is based on custom, not religious law.