By Cat Zakrzewski 

LinkedIn Corp.'s stock yo-yoed in after-hours trading Thursday as investors realized it beat growth expectations because of a recent acquisition, not due to a recovery in its core business.

The professional-networking site said second-quarter sales jumped 33% to $711.7 million, surpassing the company's forecast for revenue of $670 million to $675 million, and the average analyst estimate on Thomson Reuters of $680 million.

Excluding stock-based compensation and other items, earnings rose to $71 million, or 55 cents a share, from $63 million, or 51 cents a share, and finished well above the average analyst estimate of 30 cents a share.

LinkedIn has been reorganizing to retain corporate customers who use the site for recruitment and to better capture advertising growth. LinkedIn's clunky transition to mobile has contributed to a drop in advertising.

However, the spike in earnings didn't come from a turnaround in its core business, but instead from its $1.5 billion acquisition of lynda.com, which closed during the second quarter. In the second quarter, lynda.com, a skills-training video library, contributed $18 million in sales. That unexpectedly boosted LinkedIn's numbers.

LinkedIn incorporated lynda.com's sales into its biggest division, Talent Solutions, a platform recruiters can use to search for candidates on the site. Sales at the division increased 38% to $443.4 million.

Shares of LinkedIn fell 4.7% to $216.49 in after-hours trading after initially rallying more than 10%.

On the back of the acquisition, LinkedIn raised its full-year revenue forecast to $2.94 billion after cutting it to $2.9 billion in April.

It said lynda.com will contribute $90 million in sales for the full year, up from the $40 million it predicted previously. The company said it expects per-share earnings excluding items of $2.19 for the full year, up from $1.90, but lower than its forecast at the beginning of the year, indicating that its core businesses continue to be under pressure.

"Essentially they are lowering their guidance," said Mark Mahaney, RBC Capital Markets analyst.

LinkedIn has been overhauling both its recruiting tools and advertising segments. Overall, the company's marketing-solutions unit, which primarily sells advertising on LinkedIn properties, saw revenue rise 32% to $140 million. The company is trying to offset the decline in display ads with more sponsored posts.

However, LinkedIn continued to see declines in its display-advertising business, which contributed to the company lowering its forecast in April. In the second quarter, display revenue fell 30%, wider than the 10% drop in the first quarter.

Advertising "is the area where we are continuing to have limited visibility," Chief Financial Officer Steve Sordello said on the analyst conference call.

For the quarter ended in June, LinkedIn reported a loss of $67.7 million, or 53 cents a share, wider than its year-ago loss of $1 million, or a penny a share.

The company wants users to stay on the site longer and use it as a social platform, not just to update their résumés and search for jobs.

In addition, LinkedIn has been trying to make its site more mobile-friendly. In the second quarter, the company said, 52% of all traffic to LinkedIn came from mobile devices.

A recent American Customer Satisfaction Index report found LinkedIn was the lowest-ranking social-media site.

Premium subscribers paying for extra features gave the company particularly low scores. LinkedIn has been responding to user complaints by retooling its messaging system as well as slashing the number of emails users receive by 40%.

LinkedIn said that customer complaints about email were cut in half as a result.

Write to Cat Zakrzewski at cat.zakrzewski@wsj.com

Corrections & Amplifications

LinkedIn lost $1 million in the year-ago quarter. An earlier version misstated the figure.

Write to Cat Zakrzewski at cat.zakrzewski@wsj.com

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