It's one of the most rapid technique to build an Email List and monetize it almost instantly after at least 500 subscribers.
There are lots of fb groups, skype groups and portal where you can find Solo Ads vendor.
Personally i use Udimi primarly.
Obviously you send an email to the list of the Solo Ad vendor where theres link of your opt-in page: with a freebie in exchange of the mail as usually.
Which will you use?
What's the average opt-in rates?
What do you think about this method?