Social bookmarking sites can be crawled and indexed by the Search Engines whereas most social media platforms likes Facebook and Twitter are not usually indexed and links are nofollowed. Starting a blog and regularly adding original content will help your site. Linking should be as natural as possible. Don't buy into any of those linking packages out there as they are likely to get you algorithmically penalized.

Social Bookmarking does still work but not as well as it may have in the past. It can still be a viable way to get traffic to your site but not necessarily always the most qualified traffic. Personally I'd say to stay away from any social bookmarking tools that will automate things for you because that will be a bit of a red flag if you're too spammy with adding your links. A more natural approach to getting your pages added to social bookmarking sites is the better way to go. Just don't expect it to magically increase you rankings, conversions, etc.