Even before Twitter's initial public offering announcement grabbed the spotlight Thursday, the market for tech IPOs had been heating up thanks to a general rise in stocks.There were 12 tech IPOs in the U.S. in the second quarter, twice as many as in the first quarter, according to the latest PwC Global Technology IPO Review, released Thursday. Second quarter IPOs were also up 50 percent year over year, in terms of the number of deals, PwC said.