Let's say they do give you the platform... What do you think the care of feeding of the servers cost?
Tumblr daily pageviews: 500m; Posterous daily pageviews: 1m
Posterous would never be a billion dollar service, but it could be the type of service that generates $10m+ in annual profit.
100 servers from a competent host + bandwidth via a pooling deal = less than $50,000 per month. It wouldn't be sexy, but it'd hold.
I wouldn't keep the current Posterous business model of chasing Tumblr into a pool of red ink. I'd place a bet that users will pay for a service that will stick around. It would reduce usage, but there's a large base to monetize that would be willing to stick around.
1m pageviews a month is about equivalent to 10/second. A single app server can handle that easily. Add on a redundant app server, two database servers, and some static/queue/mail ones, and you're looking at 10 servers _tops_ for that level of traffic (and much more).
"We're really struggling here cap'n":)
Quantcast actually gives 1 million pageviews a day (and not a month), which is, yes, 10/s (and 550 times less than Tumblr).
Either Quantcast numbers are very off, either Posterous can indeed run on very few servers.
(edit: but yeah, you only need 100 servers if you're running something with "enterprise" in the name ;)
100 servers would assume media hosting for images and other files that go with a blog. That could all be kicked out to other services like Amazon, but the costs would obviously go up and I don't think it would be necessary.
At $500k +/- a year in infrastructure costs, I think Posterous could be wildly profitable. You could run a tight ship with five to ten people (throw in $500k to $1m in employee costs). If someone put a gun to your head, it could be maintained with a two or three person team.