I've seen many leaderboards used in idea management systems through the years that promote things like the top idea contributor or who has the best reputation and the like, all with the hope of driving or encouraging user participation. I won't say that these tools can't deliver value, but when used system wide and on a continual basis, I would argue they cause more harm than good. And here's why.
If you think about these boards logically, they end up becoming a promotional display for a certain number of people. For example, John has accumulated 5,000 points and will always be displayed at the top of list. No one will surpass John, and if someone does, John will still be displayed as number two. Yes, you can reset points every year and yes some systems degrade points based on some participation algorithm but the results are still the same. Because a normal user can never surpass these people it actually becomes a negative for the system overall as opposed to a motivating factor.
I would argue a better feature would be to randomly highlight individuals who have recently contributed, regardless of content, amount or value, and thanking them for contributing and participating.
There is a concept called the long tail. Chris Anderson's book tells of the story of the Long Tail for retail books and music, but his observations, conclusions and advice apply directly to collaborative innovation. My friends and past co-workers Dr. Robin Spencer and Tim Woods, have also published a paper on this topic and how it applies to ideation.
The big take away from this theory as it applies to collaborative innovation, is namely that most of the content (and value) comes not from a highly active minority, but rather from the majority of individuals who contribute only rarely, once or twice per idea campaign. By rewarding or recognizing individual quantity (with prizes, bonus points, leaderboards, etc.), and thereby ignore or even denigrate the rare contributor, we risk losing a great deal of the potential in our systems! Your recognition strategy should focus on increasing the number and diversity of people that participate instead of praising the numbers of those who are already frequent contributors.
As I mentioned earlier, it is possible to use leaderboards effectively, but only for certain types of innovation campaigns where participation is compulsory. For example, let's say that you're running a workshop on innovation, and the participants are divided among ten tables. You challenge each table to generate as many ideas and comments as they can in fifteen minutes. The table with the most contributions, wins. In this competitive case, a leaderboard, communicating the totals from each table, is useful and effective, in generating content. This approach may not generate value, but it will generate quantity.