I also think that online petitions are not slactivism because they (well, most I've seen do) include true awareness about an issue. Just tonight I signed an Avaaz petition protesting a proposed pipeline in British Columbia's Great Bear Rainforest. The petition described the proposal and explained the impact this would have on the environment. It also gave voice to First Nations' opposition.
I spoke out against #boobstagram in a blog post earlier this year.