Not clear from your question. It is not legal to impersonate someone whether in writing or on Facebook, unless you have their permission or the estate's permission. Parody is another issue and should be vetted by an attorney before you do it.
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I'm not sure what you mean by impersonating him. He is deceased so nobody could take you for being Howard Cosell. If you're just making a parody or satire of his speaking or writing style there should be no problem in doing just that; if there was, well-known impersonators like Rich Little could not have been doing what they have been doing for years on TV.