Say I have a community-driven Web site with items that can be ranked by the users. When designing the ranking system, when should I allow (or disallow) being able downvote items?
For example, Facebook only lets you "Like" (upvote) photos/videos/etc. However, Reddit lets you upvote and downvote things.
Only reason to not include a downvote button would be if children use the website or people post personal content on the website (this is the case with Facebook). For ranking items, one can surely include a downvote button.
Still, I would suggest using rating stars (e.g. 4.2 out of 5). This would avoid issue with downvote as well as there will be less chances of items being ranked same, hence better ranking. E.g. Google Play, Amazon, etc use rating stars for items/apps instead of upvotes. Facebook does not rank posts while Google Play does rank apps.
I think this depends entirely on why you want to rank these items in the first place.
If you are measuring user engagement then I would only allow upvoting or have a range of reactions that increment counters. This is the approach Facebook, Twitter, and most other social platforms take where the more popular or controversial posts gain attention based on the reactions of users. This method invokes a more positive reaction from the engagement and doesn't immediately discourage users that post "bad" content as there is no consequence for a "bad" post. It encourages more interaction and higher engagement leading to more interaction time and thus more conversions.
If you are aiming more for a community moderated environment then a points system works well especially when coupled with a points total to track progress. This is in effect gamifying the interaction with the system and encouraging content which invokes a positive reaction from the most users. This system should generally be coupled with a good set of moderation tools and a strong code of conduct to avoid things getting out of hand.
It may be a good idea to keep things simple if all you are trying to do is sort by which posts are the most useful to the user. Many services which allow user reviews such as Amazon or Steam follow this pattern and ask users "Was this helpful? Yes/No" rather than providing a like button or voting options. This gives the user more feedback on the intent of the voting system and can be useful when combined with sorting options that allow the user to view either the most recent or most helpful posts depending on what they need.
What I would do in that situation is look at the reasons for having a voting system in the first place and analyze whether it would provide any benefits to the user as well as to your project. Does it drive engagement? Does it help users find more relevant information? Will it upset users if content is downvoted and is there a potential for aggression to develop?
Here are a few things to consider:
That depends on the owner's intent. Neither is an absolute truth.
Facebook's approach is De mortuis nihil nisi bonum. Since every post in Facebook is "dead" we don't want to speak ill about it. (joking)
Reddit's approach is social choice.
