The risk of sending the UID of the user to ad networks is that it's possible an unscrupulous ad network will store and use or sell the data. However, if an ad

The risk of sending the UID of the user to ad networks is that it's possible an unscrupulous ad network will store and use or sell the data. However, if an ad network wanted to get the UIDs they can still easily get them. Just about all Facebook iframe applications use the Facebook Javascript SDK, which provides methods that can easily be used to collect the user's current ID or other information. Most ad networks require developers to load a remote javascript file on their site, and this javascript file can simply make calls through the SDK to fetch whichever information it wants.