Do share prices always drop during acquisitions?

$PayPal(PYPL.US)$ $Facebook(FB.US)$ $Microsoft(MSFT.US)$ Seeing the price of PayPal drop so heavily just on rumours they are buying Pinterest has really:

Made me want to buy some PayPal

Made me think…

Do acquisitions and rumours of such always result in a drop for the larger company? Did this happen when Facebook acquired WhatsApp/Instagram? Or what Microsoft bought LinkedIn? Or is it purely because big money thinks they are overpaying?