Do abandoned photos on photo sites ever eventually become the photo site owners property?

Like on Flickr, Webshots, there are users who have abandoned their accounts. I'm thinking like in 100 years if these website are still around and people have not logged into their accounts in 100 years or so. Will the website owners legally have a right to use the photos any way they want? I know works from Beethoven and other composers after so long become public domain. Same for photos abandoned?