In regards to the title of this thread and the question, basically its been covered.
Advertising spots are sold on their sites, this earns them money.
They also promote paysites for their surfers (see above point)
They also usually have a few links in the webmasters section, promoting sponsors, which also generates revenue.
Its not difficult to spot how they make money, go to their linklists one day & study the layout of it if you're really interested... the answers are really all right there.
Look at the page like a surfer might, you'll see soon enough.