“I included the picture of the Great Falls of the Potomac because I like it and because it reminds me of my favorite scripture verse, from Amos,” he wrote at the time, including the same quotation he posted on Friday.

Both of the social media accounts make reference to Mr. Comey’s much-anticipated book about his time in government, “A Higher Loyalty: Truth, Lies, and Leadership,” which is slated to come out in the spring. Mr. Comey was the F.B.I. director from 2013 to May 2017, when he was fired by President Trump.

The early afternoon tweet and Instagram post came after Mr. Flynn pleaded guilty on Friday to lying to Mr. Comey’s F.B.I. about conversations he had with the Russian ambassador last December. In doing so, Mr. Flynn brought the special counsel investigation into Russian interference during the election into Mr. Trump’s inner circle.

Mr. Comey’s post reflects a habit on display in recent months in which he has appeared to use a carefully chosen quotation to allude to someone or something in the news. Of course, all subtweets — the term of Twitter art for critical tweets with unnamed targets — have deniability. Mr. Comey could not be immediately reached to deny (or confirm) that he was subtweeting Mr. Flynn. He gave no further clue on Twitter.