When it first started a couple of years ago, the agreement (as it was explained to us) was basically Ars Technica gets new content in areas they don't normally have good coverage in (in this case, workplace issues for programmers) while Stack Exchange otensibly gets more exposure. The actual content creators (question askers and answerers) are not compensated in any way beyond the normal SE gamification features that kick in when a question becomes popular on the network.
I don't know if Ars Technica now decides on their own what questions to feature, but when the partnership started, Stack Exchange itself was actually deciding which questions to feature.
N.B. SE also had a Lifehacker content-sharing agreement a couple of years back[1], but I don't know if that's still going on.
[1]: http://meta.stackoverflow.com/questions/119398/do-lifehacker...
I don't think they give a f*ck that someone on hackernews happens to notice it on Ars rather than SE.
I find Ars to be one of the better web publications out there. Until theverge came along, I would say they were bar none the best.
It's 2013. Articles are text. This is silly.
Social sites including HN matter so much to these sites that many of them (not Ars) including media giants have been caught spamming them many, many, many, many times to pull the traffic, the SEO link juice and the viral spread of their shitty rewrites, rewordings, and repostings.