Currently the biggest social media sites are Twitter and Facebook. I like to reserve these as soon as I have registered a domain name.
A handy site I use to help in checking my username id is Knowem.
Knowem checks the availability of your brand name, username or vanity URL on over 400 popular Social Media websites.
You can sign up to each site manually or you can sign up to their automatic service that will do it for you. I would recommend this service if you are launching a large e-commerce site or if you are adamant that no one else uses your name.
I went through the list manually and signed up to many of the services. I had never heard of many of these sites so it was interesting to go through the list and discover new social media websites.
When you are creating your user profile at each site take a few minutes to fill in your profile details. Have your profile blurb handy so you can copy and paste it to each site. Upload your site logo or personal mugshot to each site as well. Every bit of branding helps.
When you create a profile many of these sites allow you to place a live link to your blog. Look for this option and add your link at every opportunity. You will find that your link will appear in the search engine results within 24 hours for some of the more popular sites.