As a .gov employee (at the county level), let me run through something here...
How many cell phones are there in the US? In 2009, Federal Trade Commission Chairman Julius Genachowski told the Senate Committee on Commerce, Science and Transportation Hearing that there were over 270 million cell phone subscribers in the United States. I'm going to assume that number went up in the last 5 years, but let's go with that number. I have a phone (2 actually), and I probably average 5 calls a day, which I know is on the law end of cell phone users. For easy math, let's say 4 calls a day per phone. That puts the number at 1,080,000,000 calls a day. 45,000,000 calls an hour. Now, I bet they don't have 500 people total cleared for this work, but let's pretend they maintain a staff of 500 in the building, on the system, all the time. Each of them would have 90,000 calls to listen to each hour. 25 calls a second. Every day. All the time. Non-stop. And this doesn't address text messages, e-mails, or the other communication methods.
Sometimes, I think that we, as a people, give the government too much credit for what we believe they are capable of doing. The reality, I believe, is far less. I know, they probably have computer software that targets certain words or phrases which when used sets off a warning light on a monitor so someone starts listening. Lord knows I've probably said some things that set off a light or two. Do I think that they monitor things that they shouldn't? Yep. Do I worry about it? In the general sense, like I worry about military intervention in Syria. Do I worry about my personal calls being monitored? Kind of hope they do. Bore them to death.