I've been online since the mid-90's going thru the dotcom boom and bust. I've written 2 books, one on SEO the second on Internet business building. I recently completed my 3rd book titled, "Futureproof your SEO". This book covers all the latest in SEO trends for 2013 you need to be award of. I focus my energy on Search (SEO), Social Media & Web Video. I'm always looking to help anyone I can to save you the mistakes and errors I had to learn from.
I would suggest looking at keywords used in your industry, then your customer or client journey. Then a brainstorm of these keywords to see what username/domain would be available. you don't want a domain and different social media profile - I like to use https://namechk.com/ it checks both and I use godaddy.com for simple domain registration.
I've gone thru this experience myself, going from an SEO expert into a Digital Marketing expert. I would agree this space is full and now I would have preferred to still be a great expert in Search rather than being known for anything digital marketing related.
I agree with everyone here, focus on a single niche. Just make sure you focus on a growing niche (ie. Facebook ads) rather than SEO which has had it moment. Unless you are an amazing SEO and want to stay within Search and develop yourself as a thought leader. It's always best to double down on one thing, otherwise, you have little value in differentiating yourself from others offering a similar lineup.