Really, now? I severely beg to differ, and the fact that this person feels like they do just goes to show me that there are others out there who share similar sentiments. First, let's begin with the definition of "fad:"
Fad: A temporary fashion, notion, manner of conduct, etc., especially one followed enthusiastically by a group. (Source)
Now that we know what constitutes a fad, the next question pertains to the duration of time and/or the number of people who must be involved for something to be considered a fad. Though those particular facets are subjective, there's a point at which it becomes reasonable to consider when something is no longer a fad. As for Twitter and Facebook, I can think of a number of reasons why both have long outgrown a fad status:
Twitter: First introduced to the public in 2006, Twitter boasts some 175 million users (or so they claim). Even if their claimed numbers can be disputed, the service continues to flourish to this day, 5 years later. I mean, if companies out there are willing to pay the current $125,000 per-day price tag just to have a promoted trend, then what does that -- along with the combination of all of the aforementioned -- say about Twitter being a fad? That it's NOT, that's what!
Facebook: First introduced in 2004, Facebook began as a specialized platform that eventually had the public practically begging to be let in. Since then, Facebook has amassed over half a billion followers. Families and friends keep in touch, companies keep customers informed, bands keep fans updated, and so on and so forth -- all via Facebook. Oh, and I suppose the tiiiiiiiiny fact that AN ENTIRE FREAKING MOVIEhas been made about Facebook just *might* have something to say for Facebook not being a fad. What more do you need!?
Now, I realize that the amount of money spent on a service ($125,000 per-day) or the fanfare that exists around a venue (Facebook's movie) in and of themselves are non-sequiturs when saying something isn't a fad, so it's a combination of those points in conjunction with how long Twitter and Facebook have been around that I base my points on.
Also, let me be clear that my points above have nothing to do with the potential decline of either company. Yes, I realize MySpace was a social site that once sat on top of the world, but even with its demise, MySpace certainly existed long enough to outgrow a fad status as well. People simply chose to flock to new services/sites that allowed them to be social in more approachable/enjoyable ways.
So, to say that Twitter and Facebook are a fad is to have a very narrow and skewed perception of how the Internet has evolved up until now. While there are a multitude of reasons that Twitter or Facebook could one day face the same fate as MySpace, there is just one major reason that will not be the contributing factor to bring that about: the fact that human beings are social creatures, and whether it's on MySpace or Twitter or Facebook or Reddit or Quora or any number of other broad/specialized social platforms, the innate passion we share as humans to be social will *never* be a fad.
Personally, you may not be very social; you may absolutely loathe social media platforms/networks, and I'm sure you have your reasons for that, but don't let that skew your perception to the point where you consider Facebook and Twitter to be fads. As the commenter "Aerowind" put it in response to the individual who prompted this post, that's just "silly."
If you haven't yet, it's time to learn to embrace social media -- or at least accept it and see it for what it really is, because it's not going anywhere. Ever.