No one on Splunk Answers can answer this question, unless they are employees or agents of a particular company, authorized to confirm or deny the presence of any relationship with said company and Splunk, Inc. To make such disclosures when not authorized is asking for legal trouble, especially on a public forum such as this.
Instead of asking such questions, I would encourage you to instead review published Customer Stories, slides from various Splunk Live Presentations, sessions from .conf, and possibly even contributed add-ons on Splunkbase to find the answers you seek. These are documentation of various companies publicly disclosing such relationships at a point in time and sometimes contain answers as to how they were using Splunk at that time, but again, it's not the assertion that any company presently uses Splunk, just that they did at one point.
