The Associated Press has reported on this issue in detail, noting
Peru's prisons chief, Jose Perez, said there's nothing he can do about Fujimori's social media use. "The first problem is that Fujimori doesn't directly manage his Facebook and Twitter accounts. So how can one restrict something he doesn't manage?"Well, quite. Surely the only way to really restrict his passing on content would be to ban all visitors and phone calls. Even if the government somehow managed to close down his Facebook and Twitter accounts, I don't see how they could stop other accounts popping up in the name of his supporters and reporting what he says in private conversations.
Fujimori Gets Out of Jail via Twitter, YouTube (AP, on NYT)
Alberto Fujimori en Twitter y Facebook: ¿puede tener cuentas? ¿quiénes las manejan? (El Comercio)
Yes, I am following him. There's no point in just having people you agree with on your feed. At the time of writing, Fujimori has 10,518 followers and follows just 13 accounts, including those of two of his children, Kenji and Keiko (both also politicians); other accounts connected to the party Fuerza Popular, led by Keiko; and one about the operation Chavín de Huántar which declares its resistance to the "Communist" [sic] IACHR. He has tweeted 40 times.