Reasons given by LinkedIn as to why some networks are being decreased by as much as 40% in some instances:
1. A connection closed their LinkedIn account
2. A connection removed you as a connection
The above actions will affect all of your network so it is a chain reaction. If one of your connection's contact closes his/her account it will affect your decrease your network.

ANOTHER REPLY almost immediately after:
*The acceptance of an InMail or Introduction does not add a connection.
This is also due to LinkedIn removing Industry and Regional access from the network statistics which in turns affects the number of connections you see in your profile.

The whole story here.
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