Comment by MicahWedemeyer

Exactly. Mailchimp has a very specific userbase, probably much closer to Basecamp than Facebook. If you're in that camp (serving ads, tracking biz finances, time tracking for freelancers, etc.) then Facebook/Twitter login may send the wrong message, that you're fluff. But, if you are fluff (music sharing, video game discussion, daily joke email) then social login could be very compelling to your potential users.