12 May 2026 · 11 min
Why last-click still inflates CAC in Thai paid social
Last-click is not a villain. It is a filing system. The trouble starts when User Acquisition Cost Analytics treats that filing system as a causal story about what paid social actually added.
In Bangkok we still see weekly CAC built from Meta’s last-click export, sometimes with a polite footnote about view-through. Finance books revenue from the warehouse. The two rooms argue about a gap that is mostly a definition, not a conspiracy by the algorithm.
Thai paid social sits next to Line chats, brand search, and marketplace browsing. A buyer who saw a TikTok, asked a friend on Line, then searched the brand name will often land in last-click search. Paid social looks expensive. Search looks heroic. Neither number is incrementality.
What last-click is good for
It is a decent operational log: which click happened last before a tagged conversion. Use it to catch broken pixels, to see whether a new landing page still receives traffic, to compare creative click-through inside a single platform. Do not use it to tell a CFO what would have happened if you had spent zero on prospecting.
View-through is not a rescue
Turning on view-through attribution often “improves” CAC by claiming conversions that would have occurred anyway — especially when frequency is high and the audience already knows the brand. That is harvesting dressed as measurement. In the CAC Measurement Studio we treat view-through as a sensitivity case, never as the booked number.
The holdout that survives a shared buyer
When Line and Meta share the same person, geo holdouts and user-level holdouts both get messy. A practical design we have used with alumni: pause prospecting in a small set of upcountry provinces for four weeks, keep brand search and CRM live everywhere, and compare new-to-file orders — not all revenue. Sample size is the constraint. If you cannot power it, do not fake a p-value. Write “unknown” and keep the spend modest.
Assisted search is the other quiet inflator. If branded CPC rises whenever you scale prospecting, last-click search is absorbing credit that prospecting created. Split the ledger: prospecting CAC should not get to hide inside brand.
A working rule
Report three columns, not one: last-click operational CPA, booked cohort CAC (finance revenue, chosen window), and incrementality status (tested / untested / failed to power). Teams that keep a single tile will keep inflating the story, even when they are honest people.