Twitter does not do any outright advertising but they put an incredible amount of time into their PR management, as well as making sure that their coverage in the news was kept high and positive all through 2009.

Their early claim to fame also was when they broadcasted the #sxsw Tweetstream in a hallway during SXSW 2007, after which their traffic launched substantially and effectively bootstrapped the service into the powerhouse it is today. They most certainly didn't sit on their asses hoping that the merits of their product would grant them traffic.

I am sure when they were looking to raise money they spent a good amount of time selling themselves, their product and getting known. This is not about spending money on advertising, rather, its about 'getting noticed.'