Run several search engine queries trying to find investors in all areas of the country/world. Change the searches from investors, angel investors, venture capitalists etc. to make sure that you cover all of the potential investors in the area. Prepare an email and send it to as many as possible - use the Bcc (blind carbon copy) option so that they do not know how many people you have sent it to.
I've been doing this recently and it gets a pretty good response - and from like 60% of signups they then go on to recommend someone else to sign up.
And yes, use all social media available; but use it wisely. Don't just spam peoples timelines or whatever.
Luke
Follow my Twitter: @LR_Jordan
Read my blog: www.lukesroadtobusiness.blogspot.com
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