Why would he be shady about his fb?

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I have been with my bf a little over 3 months and he will not add me to fb first he told me it was drama so I let it go then he said it was because of his ex wife who he refuses to delete btw so once again I let it go then he tells me he deleted the app off his phone and he thought that deleted the account yeah OK we all know that's not true but then he tells me he forgot his login stuff so he can't even get into his account well I just noticed he or someone changed the privacy settings so I brought it to his attention the only thing I don't get is that he seems so open about everything else I can come and go in his house anytime I please I have laid right next to him and watched him with his phone etc. so why is he so secretive about fb?