Check the terms when you register a domain on specualtion. A website design client reported concerns that one of their www domains was hijacked. They asked me to investigate a domain registered on speculation for future use. Their site was supposed to be parked, but instead it displayed a one page directory with links and advertising.

The site was not hijacked because the temporary content is within the agreement terms for hosting with their service company. The income stream enjoyed by others while your sites are waiting for launch may be characterized as opportunistic by some, yet it is perfectly legitimate if the registrant accepted those terms.

Visit the welcome message of your parked domains. Is advertising shown? You may be entitled to a percentage of the revenue. It doesn’t hurt to ask.