The ease of organizing that has resulted as a result of the advent of social tools is a staggering one that I'm pleased to be able to take part in. As a member of WSBF, the campus radio station, I've seen weeks were we, as an organization, put on multiple concerts. Social media come into this system in several ways.
First off, members of the station are constantly using social media in order to find new bands. They'll post about their favorite, a friend will recommend another, and then that band tweets about a third. This band starts getting airtime on our station as a result, and we decide to have them play a show.
We do this with email, allowing us a flexible and informal means of communication.
Once the band is booked, we start spreading word of the event through different means. We create a facebook event, mention it in our statuses, and send out mass emails and texts. The band does the same, and people spread the word to their friends who might be interested. People who are interested in the genre, but who don't know the band, can listen to songs online and find out what friends are going. We get active feedback on what people want to attend, who can't make it, and what they're doing instead (in the case of the punk show last night, the answer was generally homework).
It used to be that there were only two different ways to tell people. Mention it on the radio, and put up flyers. We still do these things, but goodness these social tools have made our lives so much easier (and shows so much more successful) that the effect is staggering.
My experience with this isn't just limited to a small student-run radio station - I've tweeted things before and had State Farm's official twitter respond, correcting my misinterpretation of their stance on turkey-fryers. Friends have solicited their friends for advice on my behalf, and I've gotten it, and it helped.
Essentially, what the development of these social tools has done is lower the cost of organizing so much that we can almost take one of those diagrams from the "Birthday Paradox" explanation, and draw a circle for each person with the internet, and then a line connecting them with every other person. 100,000,000 with
1.0 × 1016 connections between them. Damn.
EDIT: Just though I'd mention that I posted this, went straight to facebook, where a friend had posted a link to live streams of protests in downtown Cairo. Appropriate, I think.