Last time I checked very few of them relied critically on Mesos (Twitter, some parts of Netflix, and Airbnb did IIRC). Most are just test deployments or non-critical applications. (e.g. I know Ebay and Paypal are also OpenStack users because they try all the new things).

Other than those, none of those are really huge companies. I have nothing against Mesos, but I hate overhyping something when it's still clearly in an early adopter phase. The side effect of overhyping is that the tooling isn't mature/simple enough for less ambitious people so they get a bad association with the project because they tried it too early after someone implied that it was mainstream already.