Halo effects are different. With halo effects, marginal users are less interested in the raw number of additional adopters and more interested in the perceived quality of the product—both for what it can do for them and for how use of that product will be viewed by others. If McKinsey, or another comparable firm, is viewed as best-practice for strategy consulting as a result of great work with others it will likely influence the perception of their work (current and potential) with new adopters (the number of clients isn’t as important as who those clients where and how well the work turned out). Additionally, customers that want to increase their reputation as a solid firm may adopt McKinsey in order to ‘leverage McKinsey’s halo’. (I’ve thought a bit about whether conspicuous consumption obtains in the professional services world, but haven’t gotten around to writing about it yet. I think there is something there, but still working on it.)

To be sure, it isn’t a clean distinction in reality, with both effects blending, overlapping, and feeding into each other.