We tried a couple of agencies and they were both useless, they just seemed to send me very young candidates who didn't fit our basic criteria.
If you want to use au pair world you need to decide first
What your age range is, what level of English you need, if you need a driver, how long you want someone to stay, if you want someone with a degree etc.
Then you need to be quite brutal if you have lots of responses.
- Delete anyone who doesn't send you a person message (it usually means they haven't paid to use au pair world, so are less committed to the process)
- Delete anyone who isn't either an EU citizen, or from Australia, New Zealand, Canada, Japan or Hong Kong as they can't get a VISA. South Africans can sometime get an ancestry VISA.
- Delete anyone who doesn't mention children in their profile.
- Anyone who doesn't have a picture on their profile I leave in my in tray for a couple of days, as sometimes it takes people a couple of days to download it.
- Also delete anyone who doesn't fit your criteria.
I also (ahem) delete anyone who is very very overweight, or is stunningly attractive 
Also check the "willing to do housework" box as you do get some au pairs who want to do child care only.
Hopefully, that will narrow it down a lot. I then converse on email and if everything seems okay, then I send them a fact sheet with more info on my family, our area, what package we offer, non-negotiables (e.g.. they must attend language classes, they cannot smoke at all).
If they are happy with everything, we then SKYPE and I ask about boyfriends etc, if they are fussy eaters on that.
I usually have around 200 applicants, any only SKYPE around 3, so its possible to really narrow them down.
Good luck.