I spoke with my husband about this when he came home from his business trip. I was telling him how I was, for the first time in my life, using Twitter as a news source on Friday 9 January to know what was happening in my neighborhood. The prior days I looked at Facebook to get the "big picture" news stories but I didn't bother with news channels (online or TV) or Twitter. I knew it was too early to know what was really happening or what the real story was.
I asked him when the last terrorist attacks were in Paris. He was still living in Normandy and we looked online. The last major attacks (2 bombings) in Paris were in 1995. There was no Internet in France at that time. There was only the minitel. There was no social media. No Twitter. No Facebook. So it was interesting to see how all these venues were now being used during the attacks and in the days after. I, too, was using them in the way I've seen them used in other parts of the world during a crisis.
Every French website I visited had "Je Suis Charlie" on their masthead. The National Police page had crashed a few times during the afternoon. The city transit page listed all the metros that would be closed for the major manifestation/march on Sunday. From the afternoon of the attack to this day, most every website has either: "Je suis Charlie" as a banner or icon on their site, replaced their profile pic on Twitter/FB with the black square and tagline, or has their logo in black & white with a black rectangle diagonally across.
Another aspect I found interesting was how the whole country responded to the attacks visually. My husband was driving a company car* home from Nice and he said all along the highway there were images of solidarity with the attacks on Charlie Hebdo.
* As an aside: He was driving home in a car used in the filming of his TV show Section de Recherches (http://www.sectionderecherches.fr and Facebook https://www.facebook.com/pages/SECTION-DE-RECHERCHES-OFFICIEL/348738047268). Peugeot has a product placement arrangement with the show and my husband has driven cars on loan down to Nice, and returned others back to Paris. The drive is about 996 km (9 hours).
He commented that all along the highways companies (Ikea, highway and transport businesses) put their brand flags at half-mast. All along the highway digital signboards announced "Je suis Charlie" instead of with highway information. And as he arrived in Paris, one of the major expo centers (Paris Expo Porte de Versailles (www.paris-expoportedeversailles.com), which usually has colorful billboards announcing current and future expositions, were all black simply stating "Je suis Charlie." With three billboards on each side and he said it felt quite eery compared to the Piccadilly Circus-feel that part of the highway usually has.