Facebook is the least transparent ecosystem. As a marketer, I find it hard to invest dollars without being able to see or understand what percent of custom audiences are exposed to ads. For example, one can upload a file (list of emails) to a custom audience, lets say 1 M big, for FB to ingest. Voila your audience is active for targeting, but how many of those emails (users) will be targeted, who knows. FB's inventory is overpriced and untraceable.
If the digital landscape allows for transparent reporting to measure marketing's incremental lift or ROI, then that becomes the baseline. Facebook believes its inventory (users) are a cut above the rest, thus their opaque model.
FB's advertising revenue is up 59% YOY, but that is not a clear signal growth will continue at the same clip. I foresee, digital marketers pushing back as marketing moves to a revenue generating business unit. Measurement becomes the backbone of all digital initiatives, can't measure FB as granular as display.
Yes, you can't track traditional media's impact. At least not yet. Its the reason why digital marketing overall has been growing at ~30% YoY.
Facebook provides plenty of conversion tracking features, if you can't figure out your ROI on FB advertising you're doing it wrong. There are obvious reasons why they wouldn't want to provide reports on what email addresses were marketed to (Google doesn't let you do that either...).
Hey, feel free to email me at neill.silva@klaviyo.com if you'd like to talk more about targeting custom audiences. Our platform is working on tools to better segment and targets audiences in Facebook, and I'd love to get your feedback if you're interested.