UPDATE: I just realized that MB has refunded me 0.005SB, pushing this whole thing into the green :-) I'm gonna leave this post here anyway since others might be confused as well.

ORIGINAL COMMENT:
I just tested MB and after doing the math there doesn't seem to be a profit. I sent 0.1SB and received a 0.23 upvote. If I did the math right, that would be a 0.67$ vote to get 0.375 x 0.23 x 6.7$ = 0.58$ and on top of that the 0.08 cents I receive in SP, so I lost 1 cent...
If I receive 2.5x or more of the amount spent, I made a profit. When I look at the latest votes, I can see that some turn a profit but quite a few are not. What determines the return ratio? Right now it seems to be like playing scratch cards.