Revenue recogniation of search engine company??

hi friends,

wana know how would search engine companies like eg : google,yahoo would recogine their revenue ??

as came to know  they take advance from their premium customer so that their name comes first in the list when somebody search for some information ..and afterwards provide service ..

so what is the method of of recogniation of revenue in such case...

is it propotional basis or can they can consider it as revenue at the time of receiving advance from their client???

Kindly share your knowledge..  :)

thanks..