Ours just keeps growing.
No changes lower.

Yes.
Homepage -50%
Some inner pages -70%
Competitors Homepage -30%
Maybe all likes not originating from the page were deleted?
For example: Likebutton in widget on other sites "likes" example.com/widgets

There is a reduction in your likes because likes may not be real ones. Facebook is doing updates which deletes every FB account which is not active and fake. So the reason may be just that.

may not be real ones

I thought about that and was curious if anyone else experienced the same.
Why would they do this? To get likes back for their spammy actions?

Seeing a reduction here too - thought it may be old stuff getting archived.

If you bought or exchanged likes, it's normal to lose those fans. If not, then you make bad posts.

Back in September Facebook began removing fake Likes (http://mashable.com/2012/09/27/facebook-fake-likes/). Even big name celebrities like Lady Gaga lost tens of thousands of Likes so it's not surprising to see happening on a smaller scale. Another reason why Likes only mean so much!

Exchanged Likes? No
Bought Likes? No
Bad posts? No, they were for different pages on my whitehat website
My Likes are still gone and I`m getting new ones slowly as usual.

reduced like status is because people realize how bad your page is. it is an honest calling to made a better facebook update status post. make sure your post have correct grammar, and interesting status. you can communicate with your fans for a stronger bond.