Icahn wouldn't be above sending a tweet like that to raise the price before unloading shares. His goal is to maximize his return, not that of his twitter followers.
What I find interesting is Icahn filed a press release yesterday, basically telegraphing his intention to post juicy bits like this to Twitter:
NEW YORK, Aug. 12, 2013 (GLOBE NEWSWIRE) [..]
Our Chairman, Carl C. Icahn, intends to
use Twitter from time to time to communicate
with the public about our company and other
issues.
[..]
It is possible that the information that Mr.
Icahn posts on Twitter [..] could be deemed
to be material information.
I'm not saying it's a pump-and-dump. What I am saying is that Icahn (and others like him) are very careful to hide their purchases from the market while they are still purchasing, in order to prevent their own purchases from unduly raising the price.
If anything, Icahn's tweet signals that he is no longer purchasing AAPL in quantity. Now he wants the market to "catch up" to his value appraisal so he can realize a gain.
