I`ve seen this graphic posted on some other forum thread before.
There`s plenty of space for bitcoin to grow,but it`s wrong to compare it to gold.
Gold is used by the people for centuries.All the people without computer knowledge will trust gold,and believe me,most of the people in the world don`t have a PC,which means that they don`t know anyhting about btc and most likely they will never buy and use bitcoin.
It might make more sense to not take the number of people with a computer, but instead the number of people with an internet connection, which is about 3,6 billion people (and rising). That's pretty good. In the future, as people get more used to BTC, and younger people, who are more prone to adopt to new technologies, grow into BTC, I could see many, many people buying BTC instead of gold. Also, BTC historically brought a lot more gains than gold. Plus it's easier to store, hide and transport, etc...
Also, I only gave BTC 10% of gold, which is still quite a conservative number.