When it comes to branding, ideally you'll want your name/username to be consistent across all social media sites.
Whether you're creating a new site or a new product, or you want to more firmly establish the one you've already got, a great tool to check if and where your desired name is available is called KnowEm.
From the site:
KnowEm allows you to check for the use of your brand, product, personal name or username instantly on over 550 popular and emerging social media websites. Grab your name and secure your brand before someone else does.
They have paid-for plans for serious online types, but you can do a basic search for free.
For example, here's the results I got when I typed in AmyLynnAndrews:
It will also tell you domain name availability and whether or not your chosen name is registered as a trademark in the USPTO (a good start to your trademark search):