This week in social media, two Facebook-owned messenger services hit major milestones. Meanwhile, Twitter is still on the hunt for a leader with a possible announcement soon, Snapchat (maybe?) hit a Facebook-style milestone in video, and Tumblr cleaned up its act -- on mobile.

It's time, once again, for Social Media Sunday!

Facebook

Messenger: Second Only to Facebook in the U.S.

Facebook's decision to separate messages into its own stand-alone "Messenger" app, though drawing a lot of criticism at the time from users who didn't want to download yet another app, turned out to be a great decision for the company.

This week, digital analytics standard comScore released its top 15-ranked smartphone apps for July 2015 (via TheVerge). Facebook Messenger was in the No. 2 spot, with nearly 60 percent of adults on either iOS or Android using the app at least once per month.

Messenger beat YouTube (by a fraction of a percent), and was only bested, itself, by the Facebook mother ship app, which over 73 percent of all smartphone-toting U.S. adult used at least once.

WhatsApp Hits a Global Milestone, Too

And then there's that other Facebook-owned messaging app that hit a milestone this week. No, not Instagram. It was WhatsApp, which announced (on Facebook, of course) that it had now hit 900 million monthly active users.

If you remember, Facebook's CEO Mark Zuckerberg began the process of acquiring the messaging app (for the cool asking price of $19 billion at the time) about a year and a half ago, saying, "WhatsApp is on a path to connect 1 billion people."

That's why Zuckerberg saw it as such a great, multi-billion dollar deal at the time, and now WhatApp is just shy of the mark by about 100 million. Shouldn't take long.

Education Collaboration with Summit Public Schools

With all of this business success, it's worth remembering that Zuckerberg gives a good amount to charity, especially northern California school systems.

So in keeping with that personal bent, and possibly in preparation for conquering yet another market (think: Zuckerberg's other long-term "non" profit expansion effort, Internet.org), Facebook has announced it is working with a local charter school, Summit Public Schools, to engineer software to help kids learn at their own pace.

Zuckerberg decided to help -- quietly putting eight Facebook employees on the project full time a year ago -- after Summit's leader asked him for help to improve educational tools put together by the single software engineer that works at the charter school.

The announcement was made to The New York Times, and Facebook was quick to point out that it would adhere to new federal recommendations for student privacy in its development process.

What will the project look like? Well, it's still under development, and under wraps. But according to the announcement, the platform is separate from Facebook, and now being used by nine Summit schools and 20 other educational facilities. And it's free for all users.

Twitter

Dorsey's Demurral?

Just a couple weeks ago, it looked like Twitter's problems would be over -- or, at least, its vacancy at the top executive level would be filled. Rumor had it that interim CEO and co-founder of Twitter, Jack Dorsey, was ready to be named Twitter's new boss.

This week, things are back to looking pretty bad for the struggling social media service, after Vanity Fair reported that Dorsey has demurred the offer -- even after the board "begged" at one point.

He's still being considered a leading candidate, but the search may be over soon as Padmasree Warrior, ex-CTO of Cisco, and Jim Lanzone, president and CEO of CBS Interactive, are now top possibilities.

According to Re/code's Kara Swisher on CNBC's Squawk Alley, you can expect a new CEO to be named publicly soon, but after Labor Day.

'Who to Follow' Feature Added

At the same time, Twitter kept trying to figure out why Facebook is so popular, and why it's not growing. This week, as TheVerge reported, Twitter added a new section to its main timeline for iOS and Android: "Who to Follow."

The section recommends good accounts to follow based on your activity, history and contacts, and stays in-line with the rest of your timeline while you scroll. Of course, there reportedly may be "promoted" follow suggestions in the future (i.e., brands), so it's good to know you can close out the feature anytime.

Snapchat

Hits 4 Billion Daily Video Views (Depending on How You Count)

Snapchat this week just hit a milestone that Facebook hit just this past April: 4 billion videos viewed daily.

According to the L.A. Times, which confirmed the numbers with Snapchat, the metric is a little slippery when comparing different online platforms though. For example, YouTube counts a view as 30 seconds or longer, while Facebook counts a view as only three seconds. Snapchat is staying mum on how long a "view" has to be in order to register, but with a video limit of 10 seconds, it can't be much.

Tumblr

Abolishing the Endless Reblog Chain on Mobile

You know, so you can read it. Ever tried to read comments on a multiply-reblogged Tumblr post on your smartphone? With the stack of blockquote indents, it's as frustrating as reading back to the original email of a multiple-reply chain.

Tumlbr fixed that this week, with an update to its smartphone apps that make the comments a lot easier to read. Of course lots of people are upset about it, but at least on mobile you'll be able to see their angry comments on a multi-reblogged Tumblr post about it!