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.
