I think Nivi might have told Anu about PreLika convo and that's how Anu comes to know about Sneha not being with Pre. Regarding the FB, I remember there was a dialog by Pre when she broke down after rescuing Samidha from fire - `Kaash koyi Prerna aake meri Sneha ko bacha paaya'.... Something like this. So, my guess is that Anu might have came to know that Komo took Sneha to orphanage and by the time he reached the orphanage, it was on fire and he rescued Sneha, covered her in Mata's chunri and gave her to Chandrika to hand her over to Prerna. But she might have decided to keep Sneha for some reason or someone stopped her from doing it. Only Chandrika can give the backstory.
Whatever it is, mera ek aur wish ka popat ho gaya. To see Anu's guilt and breakdown when he comes to know that Sneha wasn't with Pre. KZK CVs.... It's my request to you. You really need to get some real training on how to write some high point scenes in a story. Otw, your products will turn out to be phuski..... Really Really phuski.