EL PASO — Frances Thrush sends 100 text messages a day and a total of about 4,000 a month. “When my phone service is cut off, I feel completely lost and anxious because I am not able to text,” Thrush said. She is a 16-year-old junior in high school admitting to being addicted to text messaging and could not picture her social life without it. “It’s a very quick and simple way to keep in touch with all my friends at once, I love it,” Thrush said. The consumer research company Nielsen Mobile, which kept track of 50,000 individual customer accounts in the second quarter of this year, found that Americans each sent or received 357 text messages a month, compared with just 204 phone calls.