The old Electronic Communications Privacy Act is a 25 year old law that covers privacy online, but unfortunately it was written at a time when the Internet wasn't exactly very popular (it was in its early days), and it gives less privacy for online communications than you would get in the real world for phone calls or mail.
There's a movement started by ACLU, the Center for Democracy, the Cato Institute and Techdirt (who are actually holding a Reddit AMA right now), along with a petition (which you should sign) to the White House, to reform the ECPA, and increase privacy protections for people online not just from companies, but especially from the government.
After the Snowden leaks, it has become obvious that NSA not only wants to "collect everything", but they believe they can do so without proper warrants (I'm talking about individual, targeted, probable cause warrants that are compatible with the 4th Amendment, not the "general warrants" they receive from a secret Court to spy on everyone for 3 months, and then get continuously renewed, no questions asked).
But while NSA may be the worst offender, the culture of mass surveillance, even of US citizens, is starting to spread rapidly to other government agencies, too, including FBI, DEA, CIA and even agencies like IRS, who actually believe they can read your e-mail without a warrant.
People may think they have "nothing to hide because they've done nothing wrong", but they only say that because they think the government would evaluate them in "reasonable" terms, for what everyone thinks is "wrong". But ultimately it's the government that decides whether you did them wrong or not, not you. So for example, you may think you did nothing wrong by sharing a link from a Wikileaks post online, or encouraging friends to go to a protest against the government, even in private, but the government may disagree with that, and it could get you in trouble.
And with US law being so complex to the point, that everyone is committing 3 felonies a day, you can imagine how easy it would be to put anyone they want in prison, and make them look like a criminal, especially when they have everything on them. That's why mass surveillance is dangerous.
Pervasive surveillance also creates big chilling effects, and people start self-censoring. If they know the government is already collecting everything they're saying or doing online, and can be searched later instantly, they may become a lot more careful about what they say, even in private, because they will fear how the government will react if they talk about terrorism, or protests, or even their taxes with their friends online. It also creates a chilling effect for journalists, and for their sources, which ultimately means you'll remain kept in the dark about government abuses, too. Is that the kind of society you want to live in, where any of your private conversations or habits can be accessed at will by almost anyone in the government?
So if you believe you have the right to online privacy, sign the White House petition, call your Congressmen and tell them you want stronger privacy protections, and they should support things like ECPA reform, the Surveillance State Repeal Act (H.R.2818 - sponsored by Rush Holt, most effective bill against NSA mass spying by far), the Intelligence Oversight and Surveillance Reform Act (S. 1551 - sponsored by Ron Wyden and Mark Udall), USA Freedom Act (H.R.3361/S. 1599 - sponsored by Patrick Leahy and also backed by Ron Wyden and Justin Amash).