LPL Investment Holdings posted strong first-quarter earnings largely on the strength of a larger fleet of advisors that kept busy getting investors get off the sidelines and back into active investing.

The company had $330 billion in total advisory and brokerage assets by March 31, 2011, according to the company. That is a record level, and a 16% increase from the $284 billion it took in during the same period last year. A big part of that growth came from advisory activity, Mark Casady, chairman and chief executive officer of LPL Financial said during an earnings call on Monday evening.

“I would continue to attribute this to sales growth,” Casady said. “We are also seeing double-digit same-store sales growth. That is more assets from existing clients and getting new advisors to the book.”

Advisory assets on LPL’s fee-based platforms reached $99.7 billion by March 31, up 23.1% from $81 billion from the same period in 2010. Net new advisory fees were $3.7 billion during the first quarter, compared with $1.4 billion for the first quarter 2010. That stemmed from strong new business development in 2010, and a shift toward more advisory business, according to the company.

Traditional reporting numbers also looked good. The independent broker-dealer doubled its net income, pulling in $49 million in the first quarter of 2011, up 91.7% from $25.6 million during the first quarter of 2010. Net revenue was $873.9 million, up 17.5% from $743.4 million during the same period last year.

Casady pointed to several business initiatives that helped the company enjoy a successful first quarter. Last year, it introduced third-party exchange-traded funds to the Model Wealth Portfolios business.

“By including ETFs on our advisory platforms, we avoid the irrational pricing issues associated with ETFs,” Casady said during the call. “In March, we surpassed $1B in AUM on this platform just eight months after launch, making it the most successful platform in our history, in terms of attracting assets.”

LPL also launched a fee-based variable annuity product in the first quarter that helped it win business from its existing and expanding client base. Casady did not hint at what that suggested about client product preferences.

“What I would look for is the momentum that’s there,” he said. “What you see between the fourth quarter and the first quarter is an increase in same-store sales. Generally speaking, when you have a backdrop of good markets and increased advisor activity, that typically shows up in same-store sales for us.”

There was also momentum in its recruitment numbers. The Boston-based company added 528 net new advisors during the twelve-month period that ended on March 31. The company claims that more new advisors joined LPL Financial during 2010 than any other broker-dealer in the U.S. “The new advisor classes are comprised of an increasing number of larger offices and higher average production in prior years,” Casady said.

The company also made strides in its asset custody business for registered investment advisors. Assets under the hybrid RIA platform, grew to $15.5 billion by the end of the first quarter, compared with $8.4 billion last year. It served 115 RIA firms, compared with 96 shops during the first quarter of 2010.

But the first quarter was not just about gathering advisors and assets. Its new client relationship management tool from and its portfolio rebalancing system allowed its advisors to efficiently rebalance multiple accounts.