They COULD have monetized, theoretically, but not once the die was cast.
Yes, lots of people have built CMS platform businesses where they charged money. But Posterous was probably, as you say, bleeding money, because they were not designed from day 1 to subsist on $12-49/mo subscriptions.
The choice of the "grow big or go home" model of startup results in a catch-22 when you cannot grow big, nor do you want to go home. The only thing you can fall back on is (if you're lucky) a 7 or very low 8 digit acquihire. Which is what this always has appeared to be: