Q: What SocNetWrk sites do U immediately create w/fledling start-up?

So you just captured a decent domain name that compliments your fledgling biz endeavor.
Q1. What social networking sites do you ideally want to capture that mimic/match the domain?
Q2. Or for Eg . in the case of Twitter,/Instagram (even gmail/Outlook/eBay) what username/handle/nic do you immediately attempt to acquire?
Q3. In those instances, where the matching name is already taken, is there a defacto-standard alternative name that you use?
TIA