Best celebrity Twitter and Instagram posts of the week: Kylie Jenner, Robert Downey Jr., Zayn Malik

E! reality star Kylie Jenner finally reacted to the #KylieJennerChallenge lip craze that has taken Twitter by storm while actor Robert Downey Jr. has not been able to resist the selfie on Instagram during the Avengers: Age Of Ultron promotional tour.

Hip hop star Waka Flocka Flame had Twitter in hysterics when he announced his plans to run for president during the 2016 presidential campaign while pop star Rihanna attended a wedding.

IBTimes UK looks back at all the social media fun from the last week...

Kylie Jenner responds to lip challenge:

The 17-year-old's plumped up pout has been the subject of cosmetic surgery speculation for months now but her fans have found a hilarious - but potentially dangerous - way to achieve fuller lips without going under the knife.

Thousands of teens have been using plastic bottles to apply extreme suction to their lips making them swell to ridiculous sizes. With the craze getting out of hand with some fans injuring themselves, Kylie took to Twitter to put an end to the phenomenon.

Zayn Malik breaks his silence:

The former One Direction singer had been MIA from Twitter since denying allegations that he had cheated on his fiancee Perrie Edwards and quitting the boy band in March.

However, he made a full return to the social networking site to thank his loyal fans for supporting him over the last few rocky weeks.

Rihanna walks down the aisle:

No, RiRi has not married a mystery man but the FourFiveSeconds singer was a bridesmaid at the wedding of a friend. The Hawaiian beach nuptials took place on 4/20, the marijuana celebration day.

The Bajan songstress looked rather sophisticated too, ditching the skimpy attire for a stunning lilac figure-hugging gown.

Britney Spears wins Las Vegas award:

The pop princess may have received some lukewarm reviews for her Piece Of Me residency in Vegas but the 34-year-old is clearly doing just fine as she won two awards from the Las Vegas Review Journal. Congrats, Brit!

Sofia Vergara and Reese Witherspoon new BFFs:

So, there is a new celebrity friendship on the scene and they are too cute. Modern Family actress Vergara and Oscar-winner Witherspoon have been pretty much inseparable while promoting their new movie, Hot Pursuit.

From feeding each other doughnuts to practically braiding hair, these new besties just adorable.

Waka Flocka Flame running for president:

If you need a good laugh, you can always count on the No Hands rapper. Waka, 28, announced that he will be running for office during the 2016 US presidential election.

Former secretary of state Hillary Clinton is currently a hot favourite as the next POTUS but judging by Twitter's warm reaction to Waka's news, she may have a new campaign rival to worry about.

50 Cent and Floyd Mayweather friends again:

They have one of the most on/off bromances in showbiz but the BFFs are back on. In Da Club rapper 50 headed to Vegas this week to visit the boxing champ as he trained for his upcoming fight against Manny Pacquiao.

It is nice to see The Money Team back in action but if history is anything to go by, the reunion might not last very long.

Robert Downey Jr.'s cheeky selfies:

The Iron Man star has been travelling the world with his Avengers co-stars to promote the latest installment, Age Of Ultron.

The tour hit a stumbling block when RDJ stormed out of an interview with Channel 4's Krishnan Guru-Murthy, but the actor's selfie-filled Instagram account proves he has been having the time of his life.

Beyonce meets Real Housewives of Beverly Hills' Kyle Richards:

Who said Queen Bey was too big for reality TV?! The Flawless singer bumped into RHOBH star Richards during the second weekend of the Coachella music festival and was more than happy to take a picture with the TV star.

Something says that Beyonce's secret guilty pleasure is reality TV!