In the this was mentioned, but unfortunately nobody (including me) came up with a better alternative to Facebook. So this is why the vote ended up there. It is easy to include and has a potential promotion effect no other voting system has.
I searched for a simple gallery plugin that would allow the combination of Facebook likes and "non Facebook likes", but did not find any. So I will try Facebook now to see how the SP-Studio Facebook community reacts to a contest. It might end in a desaster if only few people vote or the result is totally bizarre due to "I have friends who vote for me" behaviour. Then I will go back to a website based voting system. But if Facebook can attract a wider audience with new people who want to participate in the contests this is a big chance I don't want to miss.
But of course I won't forget the loyal community we have here. So no matter how the Facebook vote will turn out, I will try my best to add an independent way to vote next month. Either in combination with Facebook likes (if they work well) or as a substitute for Facebook (if it won't work at all).
By the way: I never vote in Facebook contests, so I understand you.

But I notice many other people do, and so I am curious how this will turn out.