It’s often difficult to find simple and straightforward figures for national library usage, especially those over any length of time.  The following will, I hope, therefore be of use for those interested in the subject.
They show good news and bad news.  For the doomsayers, we can see that overall visits and adult borrowing has indeed declined, although perhaps not at the catastrophic pace that some seem to believe.  This decline is mirrored in the reduction of the number of libraries and mobiles available and in the number of books available for lending.  
For good news, we can see that child usage has stayed the same and has indeed slightly increased recently.  A strange trend for a supposedly outmoded service.

Tim Coates, who kindly provided these charts, argued in a final slide that the points to be made were (a) “The library service has been well resourced with both capital and revenue”, (b) “Book collections have been neglected and as a result the use of books have fallen and (c) the rise of council overheads have stifled improvement”.   A video presentation by him analysing Somerset’s expenditure is also available. However, one should say that Tim’s views are not one uniformly shared, especially by librarians.  For my part, I don’t know enough to make a firm stand either way, although I would agree with Tim that an easy correlation can be made between bookfund and the resultant book borrowing.

There is something, though, that most would agree on.  We should expect the revenue and capital to fall off a cliff in 2011/12 and into 2012/13.  Whether there will be a corresponding decrease in usage will be the key to the survival of public libraries, at least at the local branch level.  A steep decline would set off a vicious circle where the decline in revenue leads to a decline in usage leading to a further decline in revenue.  This is what many fear.  It’s up to us all to make sure that it does not happen.

I have amended this commentary due to feedback kindly received.  And added paragraph (29/2/12) is in italics.

Number of libraries was already accelerating downwards
before 2011.
Spending on new library buildings and refurbishments
increased greatly.
Staff costs have largely stayed the same
Available books dipped significantly from 2001 (c. 68m)
 to 2007 (c/ 59m) then levelled of to some extent.
Budgets rose 2001 to 2006 then levelled off, with a decline
starting in 2010/11
However, the amount that Councils took from these budgets
 started shooting  up from 2005 from around 10% then to
 14% in 2010/11, presumably making the recent overall
 decline in libraries budget more noticeable.
Child usage has levelled/slightly risen.
Adult borrowing’s decline slowed by 2006/7, with
decline starting again in 2010/11
Visits increased to 2004/5 but have declined since.

From CIPFA data, charts kindly supplied by Tim Coates