mithen

I've seen it on LJ as well--my feeling is that it began largely on Tumblr (I do not know why I have this impression, I must confess!) and has spread outward to other platforms. Some of it is the tagging culture on Tumblr, the way people like to tack funny/descriptive tags on their posts--"my feels" works really well as a snappy quick tag for a post, I think.