It's funny that I came up with exactly the same idea this spring and was almost going to apply for YC funding. But I changed my mind after talking to a couple of my friends (international students who know more than one language), most of them said it's a good idea but they also felt it's not very comfortable for them to talk to strangers face to face (online). Another challenge facing such kind of site is how do you motivate strangers to actually teach each other languages while keeping their attention span long enough, say, 5 minutes. Without a good motivation system verbling will just end up being another video chatting site. I also couldn't find an easy way of generating profit besides selling ads and charging user for talking to professional language teachers. Maybe the site will build a reputation system so the best users can get paid talking to other premium account users, and site will take commission from each transaction. But in order to do that, you need to have a big user base, after all it's a chicken and egg problem.
"not very comfortable for them to talk to strangers face to face (online)" - this may be true of certain people but there are large numbers who will do based on my experience of using a text and voice language exchange site called sharedtalk.com. There are always many people there, particularly seems to be Spanish (especially from Colombia for whatever reason) and Portuguese (almost all from Brazil) speakers wanted to exchange with English speakers. Although the text chat is most popular there, the users are often very willing to exchange Skype names to use voice or video chat.
"how do you motivate strangers to actually teach each other languages while keeping their attention span long enough, say, 5 minutes." - In my experience, you do not actively teach each other the language, more just chat in the different languages and correct and explain the mistakes your partner makes in your native language. Thus, boredom after 5 minutes is not a problem, provided you get along, especially with all the different cultural differences to talk about.
I agree with most of the points you made, however, as ccarpenterg just said, it's a sharedtalk for beautiful people, people who are introverted or not confident about their physical appearances might not be willing to video chat with strangers. In the first case you mentioned, were user willing to exchange skype names before or after they had the text chat? The reason I am asking is because text chat might make a very big difference in building the initial trust between users.
This is an excellent point, I think you are right. On shared talk the number of people in voice chat is always significantly lower than in the text chat although once in text chat people often, after say 5 mins of chatting, ask to move to skype for voice chat. Also, as a learner of Portuguese, I've noticed that the Portuguese/English native speaker ratio is usually much high in voice chat than for text chat which may be to do with the more extroverted nature (in general) of Brazilians compared to people from English speaking countries.
Also you want to make sure the ratio between different language speakers is well balanced. I can see the reason behind a lot a Brazilians and Hispanics wanting to learn English, it's the most important language after all. But are there as many English speakers (mostly Americans) who want to learn Spanish and Portuguese? What are the motivations for them to pick up a second language?
No, from my experience with sharedtalk this is a definite problem for certain language learners (although not necessarily the startup since they don't need to appeal to everybody). Native speakers of English are in demand from native speakers of almost all languages. If you are a native speaker of a language popular with English speakers (such as Spanish or French) and want to learn English you are also in luck. However, on sharedtalk there are often many speakers of certain languages (such as Turkish and Chinese) wanted to learn English whilst the reverse is not true, and if, for example, you are a Turkish speaker wanted to learn Czech then your chances of finding a suitable exchange partner are very slim indeed.
The speakers on sharedtalk also vary with the time of day, so it helps if your timezone is similar to that of the countries which speak the language you wish to learn.
