AdParlor says it enabled a “large forward-thinking travel company” to run ads directed at Facebook users who had previously stayed at one of its hotels. The Custom Audience feature allows advertisers to upload CSV or TXT files — in this case, a segment of the travel company’s existing database of guests. Facebook then hashes the data so users’ identities and information are protected. Facebook runs this list against its own hashed database of users to find matches. The matches are then eligible to see relevant ads.