Prisoners are not allowed to access social networking websites (such as Facebook or Twitter) while they’re in custody.
You cannot email prisoners directly, but you can use a service called Email a Prisoner. If you send a message this way, it’ll be printed out and delivered by prison staff. Each email costs 40p and you need to buy credit to use the service.
In some prisons, prisoners can also reply and attach a photo through Email a Prisoner. Check which prisons allow replies.