As far as I know when orkut came around here there was simply no other social network with a significant number of brazillian members. Myspace was just too complex to understand, I think, and part of the annoying things of early orkut's design (you had no news, so had to keep checking your friends' and comunities' pages out to see what was going on, which fostered a whole cyber-stalking culture of checking people out) really clicked with a lot of the demographics.
I think more than by analogy that the dating market over here is also underdeveloped, and there is definitely demand for something that caters more closely to the brazillian forms of dating (which are different from the european forms of dating, and from the indian).
First and foremost, we don't have the concept of a date in the same way, and we don't have the concept of a pick-up bar/pub either. As far as I can tell, most people would be really unhappy with posting their face and names on a butcher's shop-like website, in the same way that most people wouldn't want to admit they're out there looking for dates (it makes you seem lame) while flirting a lot.
The interaction patterns between people and specially couples are different, so it's hard to say what would work over here, but outside of the gay market it certainly doesn't seem to be a traditional dating site.