It could be possible that the extension letter might have been lost in the mail. Mails do get lost sometime. Why don't you call the USCIS over the phone and speak with an immigration officer. Your call won't be transfered to an immigration officer unless you make some kind of story to the person who would come up online.
Most people say that they file an application like I-485 or N-400 for 5 months and still didn't receive any receipt notice which prompts the operator to transfer the call to the immigration officer; otherwise a call will hardly be transfered to the immigration officer. Just remember, the person who will come up online first is NOT an immigration officer. So it would be useless to tell your story to them. They know nothing about immigration, nor they have a computer to access to INS system. All you have to do is to trick them to transfer your call to next officer or immigration officer, but you don't have to tell them to transfer your call to an immigration officer. Just tell them that you have filed I-360 more than 5 months ago and didn't receive any receipt yet and wondering what's going on. This way your call will be transferred to an immigration officer.
Once immigration officer would come up online, you can ask the status of your Prima Facia case. They will be able to tell you if it has been extended or not, or if any letter was mailed out or not. And if somehow you don't be able to speak with an immigration officer over the phone, go to local INS office to know this by making an appointment online. Just wondering why do you care about prima facia letter/status when it means nothing. Are you looking to take some public assistance based upon that letter as you can?? That's the only advantage of that letter/status; otherwise INS don't seek deportation of I-360 filers for at least 15 months.
