I see your point, but there's more numbers when it comes to companies, there's a break at 7 or 8, and another one somewhere around 20. I might have fudged the numbers a littlebit, but they're both breaking points when a company needs to change its organisational structure: Starting with a flat, informal startup, at 7 or 8 is where people may feel their voice is not informally heard at (for instance) meetings, that needs to be addressed by a (slightly) more formal type of meeting. Somewhere around 20 there's a second break which is when you need to start thinking about an extra level of management (just one at first, of course). There's probably a few more that I don't know of and I would not be at all surprised if Dunbar's number is one of them.
"In my opinion it is at 5 that the feeling of "team" really starts. At 5 to 8 people, you can have a meeting where everyone can speak out about what the entire group is doing, and everyone feels highly empowered. However, at 9 to 12 people this begins to break down -- not enough "attention" is given to everyone and meetings risk becoming either too noisy, too boring, too long, or some combination thereof. Although I've been unable to find the source, I've heard of some references to a study from the 1950s that says that the optimum size for a committee is 7. Likewise, it's fairly easy for us to see and agree that a dinner party starts to break down somewhere above 7 or 8 people, as do also tabletop games of both the strategic (I prefer 5) and role-playing varieties (I prefer 7). These size limits can be overcome, but require increased amounts of "grooming".
"The chasm that starts somewhere between 9 to 12 people can be especially daunting for a small business. As you grow past 12 or so employees, you must start specializing and having departments and direct reports; however, you are not quite large enough for this to be efficient, and thus much employee time that you put toward management tasks is wasted. Only as you approach and pass 25 people does having simple departments and managers begin to work again, as it starts to really make sense for department heads to spend significant time just communicating and coordinating (and as individual departments become large enough to once again allow for the dynamic exchange of ideas that had previously occurred in the original 5-9 member seed group).
"I've already noted the next chasm when you go beyond 80 people, which I think is the point that Dunbar's Number actually marks for a non-survival oriented group. Even at this lower point, the noise level created by required socialization becomes an issue, and filtering becomes essential. As you approach 150 this begins to be unmanageable. Once a company grows past 200 you are really starting to need middle-management, but often you can't afford it yet. Only when you get up past that, maybe at 350-500 people, does middle-management start really working, primarily because you've once again segmented your original departments, possibly again reducing them to Dunbar-sized groups."
