Well, the ECB has given it [re]consideration... by excluding repos from their M3 starting in August 2012.
From a
monetary analysis perspective, it should be
checked, then, whether any of the liabilities issued
by the shadow banking system substitute
traditional bank deposits – that is, whether
they are comparable, say, with the bank deposits
included in M3 in terms of their degree
of liquidity and their risk characteristics.[footone: The extent to which these liabilities are used as money
substitutes is subject to cyclical fluctuations and dependent
on market developments.] If this
is the case, increasing shadow banking activity
would diminish the meaningfulness of monetary
indicators, with the monetary aggregates
needed for price determination in the goods
market, in particular, being defined too narrowly.
That is why, in an ever-evolving financial system,
it is crucial to regularly review the definitions
of monetary aggregates. At the launch of
EMU, the Eurosystem decided that the group
of monetary financial institutions (MFIs) designed
to capture the money-issuing sector
should include not just euro-area commercial
banks but money market funds as well, the
rationale
for this move being that for investors,
money market fund shares were close substitutes
for bank deposits in terms of liquidity and
therefore, much like bank deposits, were likely
to be related to spending decisions.
However, as monetary analysis is based on the
consolidated balance sheet of the MFI sector,
the concept of balance sheet identity dictates
that a corresponding counterpart must be
entered for each newly added monetary variable.
So if the Eurosystem adds the liabilities
side of money market funds’ balance sheets
– that is, the issued money market fund shares –
to the MFI sector’s consolidated balance sheet,
it follows that the assets side of money market
funds’ balance sheets likewise needs to be included.
This was relatively straightforward for
money market funds because they could be
subjected to reporting requirements and their
business activities essentially confined to receiving
and investing fund assets (the latter primarily
in short- term near-bank investments). [...]
The increase in significance of the shadow
banking system goes hand in hand with greater
interaction between shadow banks and commercial
banks. While an unconsolidated analysis
of payment flows between the individual
financial
sector players is of interest from a
financial
stability perspective, monetary analysis
seeks to adjust the monetary and credit aggregates
affected by the interaction, if need be, so
as to best capture the changes relevant to price
developments.
Repo and reverse repo transactions
Secured money market transactions known as
repo and reverse repo transactions, which
commercial banks conclude with central counterparties
(such as Eurex Clearing AG, which
until recently was statistically classified as a
non- bank financial intermediary), are examples
of this. Repo transactions (reverse repo transactions transactions)
were originally presented in the banking
statistics – which are crucial for monetary
analysis – as an outflow (inflow) of funds for
the money- holding sector. Yet for the most
part, these transactions inherently constitute
secured money market transactions between
commercial banks in which the central counterparty
merely acts as a go-between. Consequently,
the money supply is not expanded at
the macroeconomic level. These secured transactions
grew steadily in importance over the
course of the financial and economic crisis
owing to the high level of uncertainty in the
interbank market. They ultimately had such a
significant impact on short-term monetary and
credit developments, both in terms of quantity
and their month- on- month volatility (see the
chart on page 27), that the Eurosystem decided
to exclude them from the calculation of M3
and its counterparts in August 2012.

I don't know how the Fed deals with this. Like I said in a comment, the M-terms don't have universal definitions, when it comes to such details.
Note that the ECB has not excluded all the shadow banking from their M3 (not even all the repos), just the repos with "central counterparties".
