Vine

5 Seconds of Summer's bass player now has a viral penis -- which is not as bad as it sounds -- but yes ... his junk's being passed around the Internet after he flashed it on Snapchat.

Calum Hood learned a hard lesson (pun intended, we've seen the video) when he sent the revealing video to someone who then reposted it as a Vine for the world to see. So, now ya know, Calum -- famous naked penises ALWAYS get leaked.

Even though you never see 18-year-old Hood's face in the video ... he took the brutally honest step of admitting it was him by tweeting ... "I'm still just a teenage kid learning from mistakes ;)"

Nash Grier -- the most followed person on Vine -- billowed out a homophobic slur so loud and angry it is truly shocking.

The 16-year-old posted the video in April -- then quickly deleted it. It features a snippet from a TV commercial about HIV. Grier clearly took issue with the message.

The reason it just surfaced -- Tyler Oakley, another social media star, got his hands on it and was appalled enough to blast it out.

Grier has since taken to Twitter to apologize.

Vine star Jessica Vanessa knows how to draw a crowd ... just shake dat ass!!!

Vanessa was at a Vine meet-up in Pan Pacific Park in L.A. Wednesday, where she chatted with our photog ... completely unnoticed by the crowd ... that is, until she decided to show off her infamous twerking skills.

Once the cheeks began to move, the crowd did too ... and before long she was surrounded by a gaggle of fans.

As the old saying goes ... if you shake it, they will come.

It's all fun and games until someone gets hit in the head with a shovel ... but TMZ has learned police are investigating the infamous "shovel girl" video ... and assault charges now loom.

By now you've seen the video ... two girls are fighting when one grabs a shovel and throws it at the other ... nailing her in the back of the head as she hightails it out.

Now a rep for the Miami County Sheriff's office in Ohio tells TMZ ... they've assigned a deputy to investigate after they were contacted by the girls' high school. Maj. Steve Lord tells TMZ ... the girl who threw the shovel could be charged with assault or disorderly conduct.

How does someone you've never heard of amass SIX MILLION followers on Vine? Allow Brittany Furlan to explain ...

Our photog spotted the Internet sensation out on Larchmont yesterday with her dad when we asked how in the world she's racked up such a massive following with her six-second vids.

We won't spoil her answer -- it's really pretty simple -- but her response made her dad super uncomfortable.