Bank of America Corp. earnings today offered some positive signs, but to us, none were as positive as the bank’s massive increase in mobile banking. BofA said today that its mobile banking customers grew 45% in 2011 to 9.2 million by the end of the fourth quarter.

But if you think about BofA’s mobile growth as a share of its online banking customer base, the year-over-year increase is more impressive. BofA says it has about 29 million active users of its online banking platform today. At the end of 2010, BofA had 6.3 million mobile banking users, or about 22% of its online banking user base was mobile. By the end of last year, that share was up to approximately 32%.

And there remains some significant runway available to BofA in mobile banking. While we presume that about 20 million users of BofA’s online banking platform could conceivably migrate to mobile (although the bank will never get a 100% mobile adoption rate), BofA’s consumer and small business “relationships” number 57 million today. At least a fair portion of those consumers and small businesses are primed for the mobile channel.

The bank’s small business base, in particular, seems to me to be a linchpin. By the end of last quarter, BofA expanded its team of locally based small business bankers to 600 from less than 100 in 2010. And BofA stated today that it plans to expand its small business banking team to 1,000 in 2012. Considering that already BofA is on a march in small business loan originations, which climbed 20% last year (I’m trying to find the actually dollar amount of small business loans outstanding within the BAC earnings report — not easy), mobile might eventually find greater growth at BofA from its small business customer base.

Online banking is showing signs of slowing at BofA. Active online banking accounts at BofA only increased 1.8% in 2011 to 29.9 million. While active bill pay customers increased 4.2% last year to about 15.6 million, online-only customers of BofA actually fell a slight 0.7% to nearly 14.3 million. That is not an expected trend. Well, at least BofA has its mobile channel hopping.