The US DOT publishes "Mishandled Baggage" statistics. These lump together lost and damaged, but for example states that Delta was around a 1 out of 256 chance to get "mishandled"... so, I'm not sure if the stats on this page are really all that far off - they seem all within the range of believable to me.

It says it is normalized against actual published lost baggage statistics.

You can calculate a ratio of social media complaints to lost bags for airlines you have data on and then infer a loss rate for airlines that don't release data.

Someone can just create a Twitter account, tweet that $airline lost their luggage, and skew the statistics.. considering there's some magic maths in the background...