There is no such thing as unlimited web hosting. It’s just a marketing gimmick.
Nothing is unlimited. If unlimited web hosting were truly “unlimited,” Facebook wouldn’t need to invest hundreds of millions of dollars into new data centers. All they would have to do is buy an unlimited web hosting plan from Hostgator for a couple of bucks and reap even bigger profits!
And while the practice of offering “unlimited web hosting” is ultimately shady and deceptive, it has become so common place, even respectable, legitimate, providers that once swore they would never offer such thing are now following suit.
And really, most customers don’t seem to mind.
It’s just easier to sell “unlimited web hosting” to somebody who might otherwise be intimidated by technicalities (bandwidth, innodes, CPU usage, etc). And those who are more tech savvy, know to read the fine print (ie. terms of service agreement).
Most customers have small web sites that don’t demand much of their servers. But they will quickly encounter the limits of “unlimited web hosting” once their site becomes popular.
Bottomline: If you are building a new web site and don’t know how big it will be, you might as well start out with an unlimited web hosting plan or some other type of shared web hosting solution. After all, you can always upgrade as needed. And there isn’t really much point in buying anything more advance right from the strart (VPS, cloud, dedicated, etc) unless you are certain that your web site will become very big, very quickly (due to aggressive marketing, existing customer/fan base, etc).
If you are worried about surviving the Digg/Slashdot/Reddit-effect there are measure you can take to make even the most rudimentary web hosting solution much more robust.