Think about what kind of community the site you are looking at has and pick the ones that are most relevant to your business.
Focus on understanding why people are at that site and what they want from it. Then you can give them that and sprinkle some marketing messages in, rather than ramming messages down their throat.
Think about how you can relate to your visitors and how they can relate to you. The corporate message is unlikely to work. It’s supposed to be social!
Don’t think that you shouldn’t be on one of the more image focused sites like Pinterest or Instagram if you don’t have a visual product. Get creative and think about how you can fit your message into that arena.
Be willing to try different approaches. Brainstorm and give each one a go. you may be surprised at what sticks.