I don't have an Ancestry.com membership, but did have my DNA done through them. I have been able to make the connection via the Library posting, and the DNA match of the tree in question.
I was then able to contact the member through the DNA match. Usually with a friendly note saying. "I see you've added my entire family to your tree, let's see if we can find a connection". As these are generally active members, I have gotten responses each time. As opposed to the much closer DNA matches who ignore the messages.
One of the two times this happened, I was actually able to make a legitimate connection. The other times, it was an endogamous situation, who was putting up trees with similar names and hoping to connect later.
Scott P. Dann