Do you have any evidence of that? Have you looked at the price of money these days? 2%? 4%? Junk spread as low as 1-2? With QE the price of money is being artificially driven down to drive investment. The dirty little secret is that a lot of that investment comes from borrowing - why not if your numbers show you making 10%-20% and you can borrow for 5%-10%?
The last time the fed held the yields so low and pumped money supply so greatly was the dot-bomb. And a lot of that was shown to be driven by debt when post-mortemed.
Truer words were never spoken. True, there are many people who find Twitter eminently useful, but there are far more who can't seem to figure out what the hell it's for, and I suspect many at Twitter HQ of belonging to the latter group. There are a glut of startups right now that don't really do anything useful, but have had no problem getting funded. The investment model seems to have moven away from "where's the market going to be in 3-5 years" to "if we invest in a spaghetti website, a sailing-meets-geocaching website, a cake-delivery website, and a social media microblogging iphone app, then surely one of them will hit."
If there is a bubble it's not much to worry about since the investments are fueled by savings and not debt. Debt fueled bubbles make losses so much worse for everyone. The worst case is that some investors get wiped out (or at least what they invested was lost) and we all move on having a few good laughs about how crazy we were back then (how many Twitter services do we need when no one can decide if we even need Twitter).
The number of bootstrapped companies out there suggests that at least some people feel like raising money isn't worth it even in this climate so I don't think this time period is all bad for tech.