Upload a database and you will be able to the people who have given you permission to email as well as a "lookalike" audience. it's a great way of using email to get your message in front of the right people on social.
However, this mindset is being challenged by some marketers who argue that while it makes perfectly good sense for an email list to find customers and "lookalikes" on social, it doesn't all have to be one-way traffic. If you're looking at email just informing whom you talk to on social, you could be missing out on the trick of using the channel to grow your lists. This not only gives you more people to talk to but also to better inform whom you target next on social as a wider audience leads to a wider set of recommendations on "lookalike" customers. It's a virtuous circle.
There are a couple of tactics you may want to try, and it's fair to say that insisting on an email address might deter some participation but hopefully the end result will be worth it.
Many marketers are trying polls where only those who register with an email, perhaps to verify their vote, are allowed to cast an opinion. There are also giveaways and competitions where it seems perfectly reasonable to restrict entry to people who have given their email address -- which, again, can be used as a means of outwardly proving the entrant is who they say they are.
Another nice idea, which could be used time and time again, is a campaign around a VIP Club or some form of messaging that makes it clear what the best offers are from you or early availability of new stock notifications that go to people on your email list. Sign up and you'll be the first to know, and you'll also be in the best position to get timely offers. The fact that Facebook and Twitter can pre-populate forms ready for the "submit" to be hit makes this approach even more streamlined.
It isn't rocket science, but it just might be a new way of looking at social. Instead of using email lists to inform whom you target on social, why not complete a virtuous circle and go after new email addresses rather than a "like" or a "share." The big difference is, of course, an email address with permission to get in contact gives a brand the opportunity to talk to that person in the future. The way Facebook's algorithm works and the sheer pace of content zipping through Twitter means you cannot say the same for social media.