I usually post while I'm at work, unless I'm interested in what someone's reply is then I'll check and maybe reply at home. All of your justifications have some relevance to mine.
1. it is part of your self-styled program of continuing education.
You can learn a lot about science and religion from this site so it does have a lot of value. This is the most important justification IMO, plus sometimes I don't have anything better to do.
2. given current circumstances, this is as close as you can get to a social life.
I have a lot of friends in RL (real life) but none of them talk about the kind of stuff that's discussed on this forum.
3. a paucity of good converstation and worthy debators in your non-virtual social circle.
Yup, this is definately true for me.
4. a desire to share knowledge and help others improve their understanding of science (or religion).
Sortof but I don't think I'm supplying that much. I'm more interested in getting my views criticised so I can refine them.
5. Its become a virtual addiction.
Well, its nothing compared to nicotine.