IMHO it's a bad thing. Too many projects and services depend on github so that downtime represents a systemic risk (homebrew was unusable during the downtime last week). Sure, you have your code backed up, but you don't necessarily have your issue tracker or gists backed up.
Now using GitHub for user credentials makes things even worse. Soon we'll see horror stories involving github being down ...
We encourage all GitHub apps to use proper caching on their end. There's no good reason that a hit on a 3rd party site like Gitrep should relate to one or more GitHub API calls. Use HTTP conditional GETs to retrieve data, and any 304's (cache hits) will not impact your rate limit.
Based on its support of tags/searching, I'd imagine Gitrep is being responsible here though.
You can say the same for Twitter, Facebook and Google. There is always the potential for downtime, but users generally don't want to create more accounts. I think the advantages outweigh the disadvantages. Plus, sites that use GitHub logins are much more appealing to developers rather than sites that use Facebook or Twitter logins.
Now using GitHub for user credentials makes things even worse. Soon we'll see horror stories involving github being down ...