The most common collaterials, which I've found are:
> LinkedIn
> eBay
> Reddit
> Instagram
> Twitter
> BitcoinTalk
> Runescape
(online MMORPG games like WOW, which are easy to change, and difficult to recover)
Could we continue this list, and grow it? - it'd be very, very valuable to solving this problem.
Assuming you are talking about accounts or the like at those web sites, I wouldn't accept any account as collateral for a loan.
I don't know about other forums and sites, but if I'm not mistaken, here, on BitcoinTalk, accounts can be openly sold and bought at auctions. I never participated in any of these, so I don't know the exact procedure how they are assessed, but if the assessments are real and accounts actually end up sold and bought for the auctioned price, there should be no particular difficulty in using a BitcoinTalk account as a collateral for a loan (with a good safety margin, of course)...
So if the borrower defaults, you just sell his account with or without discount and get done with that