Yes it's a no-brainer that CloudFlare is in a prime spot to collect a lot of data on the traffic that passes through.

However are they going to turn this into an ad-tracking software, or just sell that data ? That seems quite unlikely. At this point CloudFlare has a proven business model, and becoming sleazy could only hurt them.

They wrote an article a little while ago on the matter : https://blog.cloudflare.com/what-cloudflare-logs/ I doubt that has changed much.

ps. full disclosure, former employee here.