Final update for anyone interested. Here is an image of the stats for the ad run:
Top section is USA audience, then Canada, then EU.
There were a total of 532 unique clicks on the ad.
Tom added somethingpositive.net to the referrer choices on signup, and 28 signups gave that choice.
That's pretty cool! The ad was for 1 month right?
The bid wasn't for any particular length of time. I just put $100 into my account, and ran advertising until I had nothing left.
The way it works is you bid on each of the markets separately. The amount you bid is for 24 hours of advertising, but the bids get compared every second. Originally my bids for EU & Canada were winning a lot of the time, but my USA bid was always outbid. About halfway through I tried upping my USA bid, and saw that I was getting a lot more clicks per dollar from the USA, so I paused the EU & Canada bids, and spent the rest of the money on the USA market.
Are you interested in doing this again (with alternative funding)? If so, PM me as I have an idea.