By Amara Osei

Publicis, Omnicom and WPP have cemented their status as the Big Three of the agency world in size, scale and clout — and according to Digiday's latest Media Buying Briefing, that much is undeniable. The more contested question is what happens directly beneath them.

New strata lines are forming across the top layers of the agency world, Digiday reports. The second tier of major agencies — a group whose membership depends on which consultant you speak with — can include Dentsu, Havas, Horizon, Stagwell, and even Brandtech Group, Dept, MSQ, Tinuiti, WPromote or PMG.

That list is doing a lot of work. When the definition of a tier stretches from Dentsu, a diversified Japanese network, to PMG, an independent performance shop, the category itself becomes fuzzy. Digiday's framing concedes as much: the composition of this middle rank is a matter of consultant opinion, not settled market structure.

What is actually changing in the second tier?

Entirely new formations, alliances and brand revamps are taking place among these challengers, per the report. The brief offers no revenue figures, deal values or named transactions to anchor the claim — a gap worth flagging for buyers weighing agency partners on evidence rather than narrative.

What the report does name is the exception that proves the omission. Few consultants even bring up S4 Capital, even though Martin Sorrell's Monks operation is working hard to get back on track by essentially handing over more work to AI.

That detail carries weight. S4 Capital was built as a digital-first challenger to the holdcos (agency holding companies — the parent groups that own networks of agencies). Its absence from consultants' second-tier rankings, combined with a turnaround strategy rooted in AI-driven production, suggests the middle of the market may be reorganizing around operating models, not just scale.

Why the stratification matters for marketers

For media buyers and CMOs, the implication is straightforward. The Big Three's advantage in size and clout is not in dispute, per Digiday. The differentiation race is happening below them — through alliances, brand revamps and, in S4's case, a bet that AI-run workflows can restore competitiveness.

Whether any of these moves closes the gap remains unproven in the source. The report documents formation, not outcome.

The takeaway: the agency market now has a confirmed top tier and an unsettled everything else — and the consultants who define that middle cannot even agree on who belongs in it.