BoomTown visited Twitter’s San Francisco HQ yesterday for a pie-filled tour yesterday.But I also got a chance to sit down to have a chat with two of its founders, CEO Evan Williams and Biz Stone (pictured here, right to left) about all the doings at the wildly popular microblogging start-up.For the record, Williams is the quiet one, while Stone is more voluble–but don’t let appearances deceive you, as both are ambitious and aggressive entrepreneurs.I asked a lot before I did the video–so don’t give me a hard time for not pushing too hard on it–but neither deigned to talk about the swirl of rumors about which giant Internet behemoth–Google (GOOG) or Microsoft (MSFT)–was going to get its wealthy mitts into the innards of Twitter (or not!). Nonetheless, the pair actually took time to talk about what they think is behind the surge in growth and awareness of the simple-to-use service, as well as the state of innovation in Silicon Valley.As hard a time as I have given them for Twitter’s defocus on an actual business plan, there is no question the pair have an unusually compelling product on their hands that has as many more possibilities than it does pitfalls. Both Stone and William, in fact, are going to be appearing onstage at our upcoming D: All Things Digital conference in late May to talk about all that and more, so here is a taste of what is to come.And, yes, that’s a bird–Get it? Tweet!–over Stone’s head, etched in the glass of the conference room door.(I also did a video tour of Twitter’s HQ with Stone, which is here.)Here’s Williams and Stone: