"It reeks of the same move Google did with its Buzz product when it automatically opted people in, and users recoiled against the action," said Anthony Mullen, interactive marketing analyst at Forrester Research.
"This is a direction Facebook needs to move in - your email is a proxy for your identity on the internet and Facebook want to usurp people's pre-existing email identities with their own to help drive up traffic to its site and lock users into its service.
"The problem is the lack of transparency - it has acted without asking for members' permission first."