It depends on the target keyword for the parked domain
One of my sites used to publish TONS of guest posts
I had lots of FOREX content guest posts
One of the links got parked after 1 year of so... and something strange happened
I looked at my referrer logs and that page was being hit by "paid to click" services (from US IP's)
Apparently, whoever owned the parked page was paying US-based clickers to click on my article and from my article click on his link.
Given how well Forex pays per click in the US, I suppose his parked pages were making $$$$