mmm I don't know, I don't like quite much the idea of adding a JSON datatype, I think it will be used by many not because of its intentions, but rather be misused. (Like... A LOT). I mean, it's good for things like your social media list, but that's a rather small subset of data. Tags and other stuff for example have to go in a apart table, mainly because it's easier to keep track of it later on. (Think big).
I would personally be soooo much happier with an IP, a CIDR, a timezone, or a daterange data type. These are the type of data I actually miss from PostGreSQL because they are actually pretty useful, without me having to call up functions each time I want to try to understand the actual data. Heck, to be able to define your own data structures would be super awesome!