Going mainstream means that the majority of the people is liking it or making use of it I do not know of this is ever going to happen with Bitcoin. There are just so many people that do not like to use there money on the internet.
They do not like to use their money online due to the deficiencies the previous payments options had. You had security problems with services being hacked where credit card information were stored and it was prohibitively expensive, with all the fees added in the chain. Bitcoin also, do not have charge backs.
Bitcoin is changing this with cheaper fees and increased security. < If you use no 3rd party services > Once people realize this, mainstream adoption will be a reality. ^smile^