In a word, no.

It is always considered bad form to "harvest" email addresses from one domain to another, and such practices will get you flagged as a spammer by a decent percentage of those email addresses when they receive your missives.

It also violates terms of service on most sites, in particular Facebook.

But there are several best practices for acquiring email addresses from social network connections:

• Post links to your eNewsletters on your Facebook wall and Twitter feed, and encourage people to subscribe to receive newsletters directly
• Where relevant, use petitions and other "Take Action" links to solicit email contact and other information from supporters
• When collecting contact info at live events, request *all* relevant information: email address, Facebook and Twitter names