Q:

Can we call graphic / web designers “typographers”?

Since graphic or web designers focus on typography and different ways to treat the typeface, can we call them typographers?
Graphic and web designers spend significant time on choosing the typeface, pairing fonts, making text readable, legible and even more which comes under typography. Especially when dealing with text-heavy projects.
I’m talking about those who use appropriate typographic principles and treat the text in effective way in graphic and web designing.
According to my knowledge: designing the typeface is called type design and the art of using type is called as typography. I believe this is true and some people are confused with these two terms (supporting link).

A:

Very rarely.
Any time you claim to be a professional X, you're staking your reputation on a claim to have specific, comprehensive expertise in that profession. With typography, some graphic designers can justify such a claim, but most can't stretch that far. 
"Typographer" implies professional expert. Most good designers are adept at typography, and many be typography enthusiasts, but only a minority have gained that specific, comprehensive expertise:

Expertise: Could you do the things professional typographers do? Could you do it all day, every day? For example, imagine you work for a publishing house who are thinking of developing a new unique typeface for one of their publications, like The Guardian did under designer/typographer/creative director Mark Porter. Could you:

research and make a coherent case for or against there being a business need for this from a typographic point of view?
identify what typographic properties and features the new typeface needs?
choose the most appropriate foundry and/or type designer, and explain clearly and coherently why?
oversee the typeface creation process, knowing when to demand, for example, higher or lower contrast or x-height (and why), which optional glyphs to include, what weights and variants, which languages to support?
produce or oversee the production of guidelines on correct usage of each variant?

Specific expertise, deeper than that necessary for graphic design. You'd be expected to know the finer points of typography even if they don't often come up in the context of graphic design, from the advanced things OpenType is capable of to the difference between a "point" and a "pica". No-one knows everything, but a "typographer" will love the opportunity to enthusiastically geek out about the fine detail side of typography even when they're not necessary for an active project. A few semi-random examples of topics I'd expect typographers to have opinions on but average designers to struggle with:

Appropriate uses of discretionary ligatures
The pros and cons of hinting 
Typography trends such as foundaries like Tart Workshop who use high-tech OpenType features to simulate low-tech hand-lettering
Anything to do with the history of typography, movable type, letraset...
Typography in-jokes, like, what's the most embarassing keming you've seen?

Comprehensive expertise: Do you have the breadth of knowledge of a typographer, or do you only know those parts that touch on graphic design? Can you give genuine, informed, professional-level advice on, for example: 

typography for paperback novels?
typography for low-quality paper?
typography for signage and wayfinding?
typography in cartography?
international typography and scripts? You don't need to be a linguist, but I would expect very basic passing awareness of what issues exists - for example, the importance of character encodings, or the fact that some scripts like Arabic are unreadable without correctly-applied ligature-like joins.

Most graphic designers (myself included) don't have this level of expertise, and can accurately describe themselves as designers and typography enthusiasts -  but to be a on a par with a professional typographer requires an extra level of expertise. 
Some designers have this extra level, some are close or working towards it, but most don't.

A:

Your first line "Since graphic/web designers focus on typography" is by in itself not accurate. Graphic and web designers do a lot more than focus on typography. They are also responsible for layout design, they also plan and design interaction(web designers), they also plan and develop color themes. So in a nutshell, even if you spend a lot of time with typefaces, that just happens to be one function of being a graphic/web designer.
Typographer, on the other hand is a very specialized skill dealing mostly with typefaces. This wiki has good info. 
HTH.

