SAN DIEGO (RealMoney) -- I don't agree with Carl Icahn on everything. He and I have agreed to disagree on a few things, though we probably agree on more than we disagree.

But here's something we definitely won't agree on: I believe his tweet yesterday, about an open letter he was about to release to Apple (AAPL - Get Report) CEO Tim Cook, may have signaled a top for activism.

When the letter was finally released, the reaction was almost (perhaps, ironically) as if Icahn had pulled an Ackman, building up anticipation of a bombshell but delivering what was perceived to be a dud. (Memo to them: That's the risk you take when you prime the pump.)

Icahn's tweet put the rumor mill into overdrive. For example, here's what TheFly wrote in a blurb yesterday:

"Shares of Nuance (NUAN - Get Report) are moving off lows amid speculation Carl Icahn will recommend Apple buy the company. Note that Icahn said he is sending an open letter to Apple CEO Tim Cook tomorrow."

As it turns out, in his letter Icahn said zero about Apple buying Nuance, the voice-recognition company, of which Icahn owns a lot. And he didn't suggest Apple buy Netflix (NFLX - Get Report) , either, of which Icahn once owned a lot. Nor did he suggest, as some thought, that he would urge Apple to spin off iTunes or Beats or fill-in-the-blank.

Instead, it droned on and on. (Hey, Carl. Ever heard of this thing called a paragraph?) It wasn't much different than Ackman's latest three-hour presentation on Herbalife (HLF - Get Report) .

Reality: If Apple's barely-budging stock today is any indication, investors have become numb to activists, especially when they're promoting what they're about to do. It was fun theater at first, but as companies started punching back and even ignoring the activists, activist influence started to wane.

Make no mistake about it. Activists play an increasingly important role as watchdogs at public companies run like country clubs for the insiders -- and those that simply can't get out of their own way. Many really could use a good kick in the rear or somebody shining a light on management's missteps.

But regardless of what you may think of Apple's products, or the stock, it ain't one of those kind of companies. It's constantly reinventing and attempting to move forward, even as it has rapidly matured.

The solution is not always more buybacks, especially if management isn't standing still.

The best activists are those who buy stakes, quietly rattle (for change, not necessarily buybacks) and attempt to work with management. Jeff Ubben of Value Act is among them. So, in many instances is Icahn... and Ackman. And a whole bunch of activists who sometimes simply get so caught up in their own success that they don't realize they're becoming caricatures of themselves, at which point they risk losing their effectiveness.

If you didn't get the point first time around, I think we're there.

At the time of publication, Greenberg had no positions in stocks mentioned.

Editor's Note: This article was originally published at 11:26 a.m. EDT on Real Money on Oct. 9.

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Herb Greenberg, editor of Herb Greenberg's Reality Check, is a contributor to CNBC. He does not own shares, short or trade shares in an individual corporate security. He can be reached at herbonthestreet@thestreet.com.