I cannot help but compare the 2 social networks I spend most time on - Facebook and Linkedin. While Facebook is typically considered a more 'fun' network, and linkedin way more professional, I find that the lines are blurring. A lot of my professional network is also on facebook, and vice-versa. But in terms of level of engagement, I use facebook so much more than linkedin. Linkedin has much to learn from Facebook
1. Ease of Use - Facebook and Linkedin both ask the question - What are you doing now. But facebook has made it so much easier for me to post what I am doing - through use of Tweetdeck. Sometimes I want to post a link to an interesting article that I have read on linkedin, especially as it might be more suited to my professional network. But I don't take the effort to go to linkedin and post it.
2. Fun - no doubt Facebook is so much more fun! Understandably so, given the more professional focus on linkedin. But who says professionals cannot have fun? There need to be more easily discoverable applications on linkedin, to gain the same level of engagement. I do not expect people to start posting pictures on linkedin, but reports like conference experiences etc. would be welcome
3. Applications - While Linkedin has opened its API in response to Facebook, the kind of applications available on the site do not rival Facebook in any way. There needs to be a way to encourage developers to develop more applications, such as revenue sharing/generation opportunities.