So far, it appears the goal was posting something that garnered no unusual attention, then later going back to edit it to insert spam. Seems like a whole lot of trouble for that, doesn't it?
I feel like it's gotta be bots at that point, probably doing this across a lot of sites. Otherwise, the ROI is too low for something like that.
And if they breached somebody's old email -- Yahoo, for example, has had its ENTIRE database compromised on at least one occasion -- they could gain access to a lot of accounts at once for that kind of a trick. Otherwise, it could be as simple as assuming people use the same password across sites.