I have never participated in any form or movement online (to my knowledge). I suppose I have never done so because I have never come across one that I was passionate enough about to partake in. Also, I find them quite obnoxious when I come across them in my News Feed on Facebook. I understand if your someone who is occasionally posting something, but if your someone who is constantly posting about, for instance animal abuse, then the first thing that goes through my head is “Geez, we get it.” If your someone who is constantly posting, your posts then can become over-looked and you no longer are doing any good for your form or movement.
In my opinion people start these movements for different reasons, some do it for attention, some because they have nothing better to do, etc. Those who join them most likely do it to keep themselves up to date on the issue they hold dear to them. I do not foresee myself starting or engaging in any movements via social media in the future; however, as you get older your opinions change and one day maybe I will partake. I do feel that organizations, such as the ASPCA, can benefit from these movements because they help to shed light on the amount of animals out there that are without a home. Unfortunately, I feel that movements can also make people get annoyed and tune out.
Overall, I feel that there is a science to movements online. Just as there is a science to social media marketing, advertising, etc. If you figure out how to make it work, I feel that it can be very beneficial to organizations.
JakeinPR 
I agree with your comment about how their is a science to online movements. Organizations who decide to participate in these know how to grow a large enough audience and get them involved in these movements. There are certain tactics they use and more many organizations these tactics end up being beneficial to the company. I think it is important for an organization to have some form of an online account since our society now is constantly on the Internet.
Jake, I couldn’t agree with you more when you say how annoying it is when people consistently post the same news over and over again. Once or twice, yes, but not numerous times a day. Well said!