I did:

"[I] decided to make a little tool to help me scrape the posts of my competitors and publish them to my group on a regular basis."

And

"I had created a spicy mix of public domain content, some supposedly copyrighted content from unknown authors, [...]"

I found this particularly striking:

"Usually it was the content of some authors that were difficult to track and attribute to. Sometimes it was blatant plagiarism."

So you were/are well aware of the problem. But instead of Doing The Right Thing (find out source, ask for permission) you just declare that to be difficult (and yes. It is. Full ACK).