If the increase in startups is do to economic expansion around new markets, then the overall increase in jobs can be durable in the sense that they are for viable businesses. If it turns out that the creation of startups is due to rampant speculation from capitol chasing better returns in an irrational way, then the jobs will disappear again when the market corrects.

So far, in this 'extremely reminiscent of previous bubble' non-bubble expansion, the source of capital has been, in theory, from investors who are making risk / value assessments in line with information about the business models they are investing in.

However, there is also evidence to suggest that people who have acquired excess capital but did not experience, nor learn the lessons of, that previous bubble, are in the process of redistributing their new found wealth in unsound ways.

As the previous explosion lasted about 6 years ('95 with 'Netscape' to '01 with 'nuclear winter'), and it was sustained in part by the unbridled enthusiasm of untrained investors, investing in IPOs. If this particular trend continues with simply VC/Angel money we should no in a shorter period whether or not the expansion is durable or illusory.

Check back in 2015, you'll know if it was like 99 or not.