By appropriating your site's existing niche community. And to appropriate you have to join and participate and become an authority, a leader, a resource to the community. Of course your site is designed to accomplish that already, right? Designed for your visitors, your niche vistors: one site to attract them, to hold them, to motivate them to return, to encourage them to recommend... If your site, which is your business vehicle, is not doing the above then don't bother with social media, you have far more pressing needs to work on.
Once your site is working, by which I mean converting (by your definition) a reasonable percentage of visitors and enjoying a reasonable percentage of returning and additionally converting visitors then it may well be worth while finding where your customers, i.e. converting visitors, spend their social media time and, depending on available resources including time, pick the best points (SM platforms) of interaction. Don't sell yourself or your business/site, remember that you are joining a community already with some knowledge of you. Simply join and be helpful. Give best possible answers to questions, suggest best possible resources - only include your site if it is indeed best for the query - and don't 'forget' competitors should they indeed be authoritative, etc.
And keep building and improving your site, differentiating yourself from competitors. By building your rep as an unbiased helpful considerate authority within niche social media communities you are encouraging members to visit your site for the first time and providing a solid foundation to support - and encourage - existing customers word of mouth recommendations. And within your niche market consumer social media communities you will gradually appropriate/build your very own business/site fans/evangelists.
Now, when I mentioned earlier 'don't sell yourself or business/site' it should not be taken as an absolute but as a guideline. Initially, I'd let actions, i.e. being helpful, sell indirectly while building a social community rep; consider this the networking introducing self phase. At some point it will be appropriate to drop a self-referencing link (or better yet when starting out a citation, an unlinked URL) - note: do be cognisant of the community's terms in this regard, do not break, ever (without prior specific permission, if such is available). And eventually mentioning some special, or especially some special just for members of that social media platform community, will be well received. My rule of thumb is to never mention my own sites more than 10% of the time, actual average is ~7%; on webdev fora a resounding 0%
The big largely unmentioned monster under the bed of social media marketing is that actual directly (note the word directly) attributable revenue from social media marketing/advertising is usually pitiful, some fraction of a percent. The reason is obvious: people are not typically on a social platform to buy something. They are there to interact with other people. They may complain (and it is definitely good customer service to step up and help) or they may say sweet things (and it definitely helps to say thank you) but they rarely go directly off to buy. Thus, it is usually counterproductive to think of social media in the short term (except to mention specials); think long, think building reputation and brand and fan base. Your website is the place to pitch the conversion, search is the place to attract traffic volume, other sites (backlinks) are the places to attract traffic quality, social media is the place to get a good name and build your very own 'Cult of' evangelical community.
