Same reason Google shows up from time to time. Supposedly, they're just indexing pages for search. If it IS the same as Google, you can just do site:dailydiapers.com and find pages here by context.
We use IP tracking to spot the search engines that spider us. so instead of "guest" we see when it is Google, yahoo, etc... why facebook is spidering I don't know, maybe someone is hotlinking pics or something