When I first started seeing this, I thought this could be some render because of poor internet error that time or whatever. I basically ignored it. I think its a issue which was okay few weeks ago I guess.
The issue is now resolved after disabling ad-blocker / adding site to exception list. There's nothing wrong with the site. It was working few days back (maybe 1 week) and when it stopped working, without checking things properly I thought this is something with the site.
Thanks to @Sebastian Simon. According to him,
That means, you probably have enabled some “social media blocker” or similar which blocks icons with a class name of iconTwitter or perhaps anything containing “Twitter” (same for “Facebook”, etc.). Maybe you can set Stack Exchange as an exception somehow.