I wish social media services had a trial function at the user level. I'd like to be able to follow someone on a 'try it and see' basis. Ideally that would be the default behaviour. After a week or a month I'd be prompted – "have you enjoyed following this person?" – and given a choice about whether to continue to follow them. That way I wouldn't have ended up following 400 people on Twitter and finding my stream boring and overwhelming. My stream would consist of the core group of people who I enjoy following. Same idea with Tumblr, Instagram, etc.
I think this would be good for everyone. Particularly for businesses and artists – people looking to use social media to grow their audience – since they wouldn't simply be looking to acquire new followers but to actively keep them. Everyone would need to be more entertaining, insightful, and interesting.