@JovialNickname
Sorry to sound like a dumbo as I don't use Facebook, but isn't it obvious that they were reprimanded and told that someone had reported them for their Facebook posts, and as a result they have now changed their privacy settings and culled everyone but their closest friends and family? Doesn't necessarily follow that they know it was you
I did think this, as I know several people who have been reported for something (people who have 150 to 500 'friends' on FB) and they have culled them down by a third or so. Even half. So they just have their nearest and dearest, and people they trust on their friends list.
So it could be that. However, with it being police, they would have the means and nohow to trace and track where a complaint came from. So it IS possible they may know it's you. (Ignore people saying the could not have found out. They don't know what they're talking about.)
As I said, I did think it was a case of them culling their friends, but it's funny that all 4 people have deleted you AND blocked you - that suggests to me that they do know it's you. I mean, if people do a 'cull,' they don't generally block everyone, they just unfriend. So the more I think about it, the more I reckon they know it's you.
@sowsomeseeds
Can you ask someone else who is on their 'friends list' to see how many friends they have now. If it's only 'one' less (you obviously!) then you will know that they have blocked just you, and they definitely know you reported them.
If you didn't like them, then it doesn't matter anyway, but yes, it does seem to be a breach of confidence. I'm not sure what you can do about it though.
@MagentaRocks
Police officers are regularly reminded of the social media policy and standards of professional behaviour. If they get reported it is not the fault of the reporter that they get in trouble. It is the fault of the person who thinks it is acceptable to put things like that on social media.
Yeah this. People need to stop being nasty to the OP. A person who commits an offence is the arsehole, NOT the one who reports it.
So just stop it! 
Funny how some people think the police are Gods to be looked up to, like their shit don't stink. Like doctors, nurses, teachers, dentists, and other 'professionals,' they are far from perfect, and some of them are capable of some very unsavoury behavior!
Bollocks to people thinking you're a 'grass' OP. Some people have the mentality that reporting people, or grassing on people, is a worse crime than stealing, or benefit fraud. Ignore them.
The behaviour of these 'police officers' is terrible, and they needed calling out on it, and reporting. You have done nothing wrong. They are behaving like they are, because of their guilt and shame.
Make no more attempts to contact them. You are well shot of these people.