Is the database full of people who opted in or skimmed from other sources?
Is the database been segmented based on knowledge about their interests?
How are you hoping to monetize this list for your company?

If these email addresses are from people who have expressed interest in your product and gave you permission to send them follow up emails, then what you have could be quite valuable. How you use it would vary depending on how your systems are set up.

Anything less (they did not opt in to get messages, they aren't all potential customers, etc.) reduces the value (sometimes significantly, to the point where the list could be valueless). Answer the questions asked above by Jay so we could have a feel for how valuable the list could be and we can help advise how to use it. In addition, a good piece of info we could use is how you got the list.

Short answer: It depends on where the list came from and what you know about the people on the list.

You can always do a pilot-test in which you take, say, 500 random names for each approach (every nth name) and see which would generate a better return on investment when applied to the whole list.

Another vote here for the origination of the list: did all these people double opt-in for an offer from you? Or is this a list you've acquired and with which you have yet to build any kind of relationship or affinity?

both.

Email can drive direct sales.

Facebook custom audiences can tell you more about these potential customers (demographic data facebook provides about your custom audience) as well as enable you to remarket tp these same people with ads.