I have been playing with this on my site. Just to see if I could get into the private tags. You can see here at
Yoddle Tags(beta). When you add a favorite, you must add your tags as well as choose to make this link private or share it. I like the idea because people find the strangest things on the net. I will then feed these links to the spider. Blah. Blah. I also want to incorporate the tags into search results. My question is. Why sign-up to this service if you were not going to share some favorites with the others. The answer to your question is that the engines will find anything that is published for public view. If you are concerned about hidding some of your favorites then if the service does not allow you to edit or hide your info. Find another that does.