The other day I was talking with an acquaintance who told me that had he created a profile on as many different social media sites as possible and posted all of his announcements to each site. The result had been that he had pushed himself up the Google rankings to page 2. That's a pretty decent result, but at the same time I had to wonder if being on every single social media site was really effective for him in the end, in terms of actually engaging people on those different sites.
I tend to feel its better to spend time on a few sites and focus on developing relationships. But I also recognize that for my acquaintance what might matter the most is the SEO benefits of being on all of those sites. Afterall, they clearly provide some benefit in getting him to stand out from his competitors, although I suspect, that doing some SEO on his site might be even more useful.
I think the reason I prefer a few sites is because even if I'm connected to all those sites, I'm not meaningfully connected to any of the people on those sites, unless I can somehow manage to monitor all the conversation and respond to it, as well as make comments of my own on other people's news. Doing that for every site is time consuming. Even just keeping track of three or four sites can be demanding on time, especially if you are cultivating active relationships.
In the end it comes down to why you are on the sites. Are you on a site because you want to engage with people or because you want some other result? But the reality is, with social media sites, you ignore the social norms of such sites at your own risk.