I think you have it right.
There is not a lot of detail provided about the two studies, however it is my understanding that in the first study, for men, the amount of time spent on facebook was not directly related to self-reported symptoms of depression, rather (for men) only that time on facebook which was spent engaged in social comparison was directly related to self-reported symptoms of depression. My take-away is that men spent an amount of time on facebook engaged in other activities than social comparison; Time spent on facebook doing other things (possibly, as you mentioned, debating politics) was not related to self-reported symptoms of depression... only time spent in social comparison had that affect.
In the second study, it was learned that for both genders, social comparisons "up", "down", and even non-directional were mediating variables; All types of social comparison could make people feel bad.
