Visit Google today and you'll see they blacked out their logo, and added a link to their anti-SOPA/PIPA campaign. Cool. Not quite as strong a move as Wikipedia shutting off their entire English site, but, fine.
Except that everyone else is asking people to call their Congressperson. Instead, Google is asking you to sign an online petition. This is a huge letdown.
Online petitions do not affect national political discourse. Maybe Change.org is helping local campaigns get traction, I don't know, but their and everyone else's national campaigns are for show. They're building organizations' email lists.
In fact, a study by the Congressional Management Foundation that surveyed Congressional staff found that over 50% of them thought that "most advocacy campaigns of identical form messages are sent without constituents' knowledge or approval". Not just that they don't have much impact - the majority of Congressional staff assume form messages from citizens are meaningless lies.
Calls, on the other hand, get through to offices in a way that emails, contact forms, and even letters do not. The only thing better you can do as a citizen is visit your Congressperson's office in person. Calling and visiting are hard. They're uncomfortable. Fewer people will do them.
And that is why they are effective. By doing it, you convince your Congressperson's office that you mean business, that your vote is in the balance, and that you're probably the sort who'll convince a bunch of other people to hang their votes in the balance too. You're making yourself a representative to your representative.
Email lists can have value - but now is not the moment for making them. This is the moment where, if you've already got one, you use them to get people to make calls.