I am NOT a "Guru" but i do have the answer that your looking for and I will warn you now, its most likely not the answer that you are looking for or wanting but its the %100 truth.
I will only speak about facebook as that is what i know best. so to answer "“how do you calculate the ROI of facebook" I say this.
Facebook marketing is a lot more complex then adwords or most other pay-per-click type traffic. It is also totally different then getting buyers from google search as when people are searching for a product in google search, they are most likely looking to buy.
On the other hand, when people go to facebook, they are looking to browse and are very seldom in the "Buy now" mode SO, if you try to sell them you stuff on facebook ads, there's a great chance that you will lose all your money.
On the other hand if you create a following on facebook via. a fan page with targeted people that you got throw facebook ads then you CAN and will be able to get a great ROI IF you engage properly with them "DAILY" and don't try to always sell them your stuff.
True, getting a ROI from facebook can be tricky but if you either take the time yourself to learn how to get fans and engage them and so on or buy a course that teaches you these things (which I DO offer), then you will indeed enjoy that elusive ROI you are searching for.
Let me leave you with one last thing, do you REALLY think that companies can measure the ROI of a T.V., Bill Board, or Radio ad? NOPE! but last time I checked, all three of those media outlets are still getting advertiser.
If you are still not convinced that your company can make a positive ROI from facebook, contact me directly and I would love to help/work with you.