NEW YORK (TheStreet) -- Mark Zuckerberg isn't done shopping.

At Facebook's (FB) - Get Report F8 developer conference in San Francisco this week, CEO Zuckerberg made clear his company isn't just one downloadable application, it's four: Facebook, Facebook Messenger, Instagram and WhatsApp.

As the whiz kid discussed the monthly active users, or MAU, for each application and their prospects for growth, he underscored one over-arching theme: The family of Facebook apps would grow. In other words, Facebook is shopping for companies.

Instagram cost Facebook $1 billion in stock, and WhatsApp required an outlay of $19 billion, again mostly in stock. 

When you look at the rest of the app charts, what else is up and coming that Facebook would be willing to buy meeting its criteria of having the capability to grow into a billion users over time?

To me, there's one service above all that stands out: Pinterest.

Pinterest is massively popular and totally different from anything out there. It is gathering people's interest graphs. That's something that is really unique compared to search, Facebook or Twitter. It's probably the next best thing after search for getting at "user intent," which is the secret sauce to unlocking billions of dollars in ad revenue.

What else is out there?

Well, there was that rumorSpotify would be acquired next week for $14 billion. Sean Parker was an earlier investor in Spotify and Facebook. The thinking is, maybe Zuck figures he needs a beachhead in music and Spotify is a service that could grow to have a billion users. It could also help diversify Facebook's revenue away from just ads.

It's no secret that Facebook tried to buy Snapchat last year for $3 billion. Snapchat looks very smart now for turning that offer down, even though the herd derided the company's decision to do so at the time. The latest Snapchat financing was rumored to have been done around $20 billion and usage has been insane. Snapchat could easily get to a billion users over time.

It's also clear that Facebook -- at one time -- thought about making a run at Twitter (TWTR) - Get Report. Why not do that again? It would be the most expensive deal of any of the ones mentioned above but there's definitely something unique about the Twitter broadcast offering which Facebook doesn't possess. That became even clearer Thursday with the launch of Twitter's Periscope service. I suspect that app is going to be massively popular, especially with teens.

But here's why I still think Pinterest is a better target. In 2014, it was estimated that Pinterest's MAUs were in the ballpark of Twitter's. The usage stats for the app have been impressive.

Yet, only 28% of U.S. adults on the Internet access Pinterest. In Europe it's even lower with only 13% on the service. That means Pinterest still has tremendous growth potential and still hasn't done really anything to generate money.

Can you imagine what Facebook could do combining its knowledge of its users across its various properties? What do users pin on Pinterest and how does that relate to where they live and went to college? What does that same user do when on Instagram? It would be a big data goldmine.

What would it cost? Re/Code reported recently that Pinterest had raised a new round at a $11 billion valuation. It would likely cost Facebook double that to buy the company.

Pinterest doesn't have as many MAUs as WhatsApp did, but they're all in the U.S. versus overseas and are therefore more valuable. Facebook would likely want to do an acquisition with stock. Why wouldn't it? Facebook can point out to Pinterest how the value of the stock both Instagram and WhatsApp took has only increased over time since those deals happened.

Zuckerberg has done a fantastic job on M&A strategy, both large and small. Pinterest would be the next big trophy for his wall.