They got ~10 top grade Google vets. Assuming they each have to stick around for 3 years,
$15MM/(10*3) = $500K/year each.
And the $35MM in stock = 0.54% of Facebook. Divide that by 10, and you get 0.05% per head, which doesn't seem extravagant at all.
It's the FF employees who are bearing most of the risk in this transaction, in the off chance FB isn't actually worth $6.5B.
Now unlike Twitter, friendfeed is not made of a bunch of marketers following other marketers. If you follow someone on Tiwtter you get something like "Thanks for the follow. I am expecting to learn from you. Check my website...".
Also The discussion around a particular shared item on FF is easier to follow.
Also, arguably Bret and Jim were the leads at FriendFeed, since they came up with the original idea. AFAIK, they approach PB and Sanjeev as investors, and the latter figured that not only would they invest, but they'd sign on as cofounders.