In their TOS, Google doesn’t state that it is wrong to email other Web masters and ask them if they would be willing to trade links with you. But they do say not to over do it and that the best way to gain incoming links to your site is by adding content to your site that other people will want to link to.
Of course, another way to gain links is to link to your site from your social media accounts. Google knows you do it, and they want you to. If they didn’t, Google+ wouldn’t exist. But it is important to remember that Google makes the rules. They don’t disclose specifics about the value of one link over another. But one incoming link from a dedicated reader’s site is worth much more than you posting a link to your site on Twitter.
However you gain incoming links to your site, and Simple Business Video Marketing suggests offering people great content they will want to link to as the best way, make sure you follow good SEO