I guess these days most Facebook/G+/Twitter users are logged in from their smartphone , so you'd implement it as a mobile app that gets API access via the standard mobile frameworks. The UI flows for this are highly optimised and very slick. Once you have API access to read/write the users profile picture, your app can just wake up from time to time and check if the users profile picture has changed. If it did, download the highest resolution available, rewatermark and reupload. 

The main sticking point I can see is that the user might end up losing comments or likes on their primary photo, which would upset some people, and they might end up with duplicates if the old one was not erased. The Facebook API docs are notoriously poor - it's unclear to me whether an app can edit a photo after it was uploaded, or whether it can only create new ones (deleting photos requires whitelisting by Facebook).

To read the users watermarked address requires no API access or account, though.

Probably you wouldn't want to watermark an actual Bitcoin address or key. The capacity of social network photos to carry stegod data is very low due to the incredibly high compression they go through. More likely you'd encode a very short URL which contains a payment request and then users would rotate their key from time to time at the hosting site.