This is part of the 1-4-1 law in Hong Kong that makes prostitution legal if it is 1 woman, working in 1 apartment (sort of like the city council resolution of last month). Now the reality is that there is "organization" (read into that what you will) The websites are put out by the "organization", and you often find more than one of the woman advertized working in the same building. And in this building you usually find a "dai lo" to make sure everything goes right.
Here is an English version of this style of website:
www.happy161.com
There are also others, but they are in Chinese. BTW the abriv ML means Make Love and is similar to FS.