No it's not. I work in open source and very rarely use github. git is a standard tool, github is only a polished frontend that seems to be a standard amongst web developers.
Yeah, there, are, um, a lot of them.
It's really not. In the last 3 places I worked at we all used Bitbucket because their pricing is more reasonable and it integrates nicely with other Atlassian products like Jira.
Ok, so Bitbucket is also a standard, critical tool.