Me and my sponsor we have never met but we are friends in Facebook since 3 years and we been talking a lot. And he decided to meet me and invited to his country The Netherland. And I have already applied for two times but I got my visa rejected just because embassy didn't have proof of our relationship. But I have only written that we are friends since 3 years in a letter. Now how can I sort it out and what information should I give to embassy so that I can get the visa?
A Bewijs van garantstelling en/of particuliere logiesverstrekking would be the most logical choice. It would establish that you have a bona fide invitation and that your sponsor is serious about hosting you.
Note that, from the perspective of the consulate, the situation raises many questions and I would not assume that the genuineness of the relationship is the only issue (Schengen refusals are quite broad and only use standard formulas so I am not sure how you drew this conclusion). But the Bewijs van garantstelling is in any case a good thing to have for this types of visit.