Does The Lord of the Flies' ending indicate that Golding is pessimistic or optimistic about the future of society?

Interesting question.  First, it helps to have some
insight into the author's motivations for writing a work of literature.  Though this
isn't always possible, we do know that Golding, in a publicity questionnaire, described
the theme of Lord of the Flies as
follows:

The
theme is an attempt to trace the defects of society back to the defects of human nature.
 The moral is that the shape of a society must depend on the ethical nature of the
individual and not on any political system however apparently logical or respectable.
 The whole book is symbolic in nature except the rescue in the end where adult life
appears, dignified and capable, but in reality enmeshed in the same evil as the symbolic
life of the children on the island.  The officer, having interrupted a man-hunt,
prepares to take the children off the island in a cruiser which will presently be
hunting its enemy in the same implacable way.  And who will rescue the adult and his
cruiser?

It is clear, from
this statement, that Golding believes that humans are inherently savage--and that the
presence of a political system is not a sufficent means of shaping people's fundamental
nature.  In other words, the "defects of human nature" are what cause problems in
society, and those defects, Golding would say, are present within us all.  (Golding
chose to use children as his characters to show that even youngsters, who haven't been
"corrupted" by society to the extent that adults have, are still susceptible to the
descent into savagery.)  Further, it's important to note that Golding's experiences in
World War II shaped his view that humans resort to savagery in certain conditions. These
things considered, it's relatively safe to conclude that Golding is quite pessimistic of
the future of society, since he believes that we are all fundamentally
flawed.

Aside from the irony regarding the boys' rescue by
officers on a warship, we see irony in the fact that Jack, who fought for power
throughout the entire novel, becomes suddenly unwilling to accept responsibility when
one officer asks who's in charge.  Here, Golding effectively reduces the warrior Jack to
a small, meek little boy:

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A little boy who wore the remains of an
extraordinary black cap on his red hair and who carried the remains of a pair of
spectacles at his waist, started forward, then changed his mind and stood
still.

Instead, Ralph, who
considered giving up being chief because of Jack's power and growing following, assumes
responsibility for the others. Not surprisingly, it is Ralph who understands the scope
of the situation--and it is Ralph (along with others) who is left weeping as the novel
closes:

Ralph
wept for the end of innocence, the darkness of man's heart, and the fall through the air
of the true, wise friend called
Piggy.