I was told Ruby on Rails has limitation in scalability. What exactly is hindering RoR to scale up (is it true that Twitter switched from RoR to Scala)? Any solution or active projects to resolve scalability issues?
Nope Twitter didn't switch. They still run Ruby on Rails. They also use scala as they run rails under the jvm AFAIK. If you really want the skinny google is your friend here.
I was told Ruby on Rails has limitation in scalability. What exactly
is hindering RoR to scale up (is it true that Twitter switched from
RoR to Scala)? Any solution or active projects to resolve scalability
issues?
Well, if you have a specific issue bring it on, I am sure you will get some takers and help here. As well you can have scalability issues in any framework or language, it just depends what you are trying to do and also how it was coded. I would guess that a very high % of rails apps never hit the limits of what rails can do.
As does everything else, I would say.
A reasonable practice is to build you app, then profile it when it becomes slow. See where the speedups are: maybe it's MySQL settings, more RAM on the servers, more hardware/instances, tighter Ruby.
If, in your profiling, you identify a routine - or subsystem that you really can't get speed out of, figure out how to write it in something else. Maybe you - as Twitter has - spin off to another language for parts of your app. Maybe you write a gem in C to get that speed boost. Maybe node.js, for high concurrent connections in a special case.
There's also knowing what Rails is good at, and what it's not. For example, this weekend a client and I were talking about how to implement a live widget (ala Facebook's news feed). We talked for a little bit about how Rails doesn't make that kind of thing easy, and ended up looking at http://www.pusher.com to provide that functionality in the app. However, I believe that knowledge comes from knowledge/experience of how the web works, known-how on the Rails side, plus knowledge of your own application and goals.
In short: premature optimization is the route of all evil (except when
it's not) 
Hope this helps, _Ryan Wilcox
I think this is a question better directed to the FUD-merchant who told you this.
--Matt Jones
jcBAM
Ruby on Rails is a Killer App and Disruptive technology.
Much in the same way other open source frameworks, operating systems, web servers, databases offset the market by enabling the developer to not be locked into a single vender.
#Avoiding_vendor_lock-in_for_computer_software
Twitter suffered a couple brownouts during a time of exponential growth three years ago. The kind of growth you and I should be so lucky to one day have with our websites. =)
If micro$oft had it's way it would get rid of anything in it's way to ensure it's place at the table for as long as it can. The Ruby on Rails framework goes into direct competition with M$ current line of developer products and frameworks. It would make sense that during the brownouts they would inject fud into the blogosphere to protect their current interests.
A more famous tweet from the co-founder of twitter during that time:
Once again mind you that this was three years ago.
So the question is does Rails scale?
The answer is undoubtedly YES.
It's just a framework why wouldn't it? Frameworks have very little to do with scaling.
Since it's an open source framework there is nothing stopping you from digging deeper into the low level aspect of it and tweak it to your sites specific needs. Chances are you will never be in that position. If you are, congratulations, I look forward to reading about it in Forbes.
As it's been mentioned I would question the qualifications of the person who gave you this assessment as they either have no proficiency in how a full stack web framework works or has an interest in selling you on a vender lock-in product.
