You would think that with 1.155 billion active users by the end of last quarter, Facebook would be raking in the payments revenue.

You would be wrong.

The company — which today AllThingsD disclosed will reboot its payments strategy with a test of a PayPal-like service starting next month — has been struggling to keep its payments business on pace with its advertising revenue.

Last quarter, Facebook generated about $214 million of payments revenue, around 12% of the company’s total revenue. On the surface, that’s not a bad number. Year-over-year, Facebook grew that revenue line 11%.

But scratch the surface, and you can see why Facebook, presumably a growth company, is actively trying to figure out payments. On a quarterly basis, revenue growth was effectively 0%. That compares to advertising, which in 2Q grew 28% on a quarterly basis and 61% on an annual basis.

Payments is clearly not keeping pace at Facebook. In the first quarter of 2012, 18% of Facebook’s total revenue came from payments. However, the percentage of payments revenue has declined since to the 12% recorded last quarter.

There’s another arguably more disturbing payments number out of Facebook. The company simply cannot get the average Facebook user to give it more payments revenue. The average Facebook user gave the company $0.19 of payments revenue last quarter, down from $0.25 per user as recently as the last quarter of 2012. Not surprisingly, the opposite is true of advertising per Facebook user. Last quarter, advertising revenue per user climbed 32% compared to the same quarter in 2012.

Now Facebook wants to PayPal-ify its massive user base, but analysts say this won’t be easy.

Forrester’s [Sucharita Mulpuru] said she’d be surprised if the social network has more than 10 million credit card numbers on file in the U.S.

“Nobody trusts social networks with their financial information, and they are certainly not going to trust Facebook,” Mulpuru said. “Maybe they have a few million people that have bought something on things like FarmVille, but that does not a network make.” …

“PayPal is well-suited because they have more than 100 million accounts with real people who have trusted them with their information. To be a latecomer to the game in something as complex as payments, they would be better off buying Square,” she said.

We are not sure that buying Square will solve Facebook’s flagging payments. But we are sure that they are flagging.