There are many sources that are available to you which will help you determine the networks that your customers (target and existing) are frequenting:
Your SCRM may have the capability to mine your contacts for their social profile information. If not, there are some third-party e-mail add-ons that have this social discovery capability, as do the standard search engines (Google, Bing, Yahoo).
Search for both individuals and companies on the networks themselves. For example, having a company presence on LinkedIn will also show you employees of that company who are also on LinkedIn.
Visit their company websites and look for social network icons that will allow you to connect with the company. Such icons can generally be found on the website's home page.
Ask them. When all else fails, ask your customers if they are active on the social networks, and if so, which ones. This would also be a good time to ask them how they would prefer to be contacted by you...