Obviously you have more knowledge of Posterous than I do, but generally a founder leaving tends to have negative implications (it scares potential investors, damages momentum).
I use both Tumblr and Posterous and from a user perspective when Posterous pivoted to be focused on "spaces" it seemed like the decision was panic driven. The reason that Tumblr was succeeding over Posterous wasn't due to the feature set of Tumblr but rather the community that Tumblr had.
Posterous had advantages over Tumblr and in many ways it's the better blogging platform, but rather than playing on those strengths and improving them Posterous instead went after the social aspects that Tumblr was succeeding in without fully appreciating that it couldn't just take a social feature set and foist them onto it's user-base.
I'd guess somewhere between $5m-$10m.
Given the number of employees and the amounts they've raised I guess their burn rate was around 250-300k/month (that's probably on the large side). Their series B was only raised 6 months ago so based on their burn rate I'm assuming they've still got 3m-4m of that in cash so that would be the lower bound for the acquisition.
Given the company probably had 12 months of cash left I suspect the Series B investors would only have agreed to a sale if they got all their money back (they'd have liquidation preference anyway so they'd get their money back first), so that probably means a lower bound greater than $5m (the size of the Series B).
Anything over $10m would be high for a talent acquisition so that's probably a decent upper-bound.
Liquidation prefs are rarely exercised, at least when the acquirer is clueful. (The company spending the money wants the incoming employees to be reasonably incentivized, often that takes the form some delayed payout. They really don't care if the investors make their money back or not.)
It's usually among the first things to go in a negotiation.
I doubt the investors made a return.