We've been using NameChk.com at Konnekt Digital Engagement for years now whenever a client comes to us wanting to launch their brand in social media or when we've got a new project like 15secTech we want to get off the ground. This web service, which has an associated iOS app available, lets you see if your desired usernmane or vanity URL is still available at dozens of social media and related sites, including Facebook, YouTube, Twitter, Pinterest, and many more.
One note of caution, though: if you search a name that is not technically permitted on a given platform (e.g. Twitter handles longer than 14 letters or containing a dash or other banned characters) then NameChk will show that name as AVAILABLE - because no one has taken it - because no one can ever take it. So make sure you know what's permitted on the platforms you're interest in and then hop over to NameChk.com to see if your dream name is still around!