For the first time since late Oct. of last year, LinkedIn (NYSE:LNKD.DL) shares closed below the vaunted $200 level on May 5. As if the nearly 25% sell-off following its earnings call -- and more precisely, its forecast for the balance of 2015 -- wasn't causing enough angst among shareholders, there might be more bad news on the way.

As noted in an article following LinkedIn's announcement, 2015's Q1 wasn't a complete dud. In fact, LinkedIn impressed with continued member growth -- expanding faster than Twitter and dwarfing the tweet-master in size, despite targeting a more niche market -- along with improved revenues and exceeding internal and analysts' consensus expectations. In the long run, LinkedIn's stock sell-off could prove to be a great opportunity for value investors. However, there are a few areas that could drive LinkedIn's share price down even further. cost how much?
By most accounts, LinkedIn's deal for online job training site was well-received. As per LinkedIn CFO Steve Sordello, the market for online professional training solutions could be as much as $30 billion, meaning the $1.5 billion it plunked down for could turn out to be a relative steal. But digging a bit deeper uncovers a couple of concerns investors would be wise to consider.

In 2014, generated all of $150 million in subscription sales. For a 20-year old company valued at $1.5 billion by LinkedIn, $150 million isn't exactly earth-shattering. Naturally, with LinkedIn's scale and alternative sales opportunities like advertising, Sordello and CEO Jeff Weiner are counting on bumping's sales results up significantly. But when?

This year is expected to add a meager $20 million to $25 million to LinkedIn's topline. At the same time, stock-based compensation is expected to jump -- again -- thanks to an additional 3 million shares will be issued shortly, and Sordello estimates about $15 million in increased costs associated with "experimenting" with different ways to go to market with its new-found asset. It could be a long while before investors see a return on LinkedIn's investment.

Speaking of costs
Whether it turns out LinkedIn over-paid for or not, it will further impact what are already staggering expenses. Perhaps the biggest reason LinkedIn was forced to dramatically alter its earnings forecast for the balance of this year -- earnings per share guidance was lowered from an expected $2.95 per share to $1.90 -- was sky-high costs.

It didn't help that display ad revenues declined 10% in Q1, or sales from LinkedIn's European operations dropped significantly; but it was the costs associated with customer acquisition and ramping up personnel that hurt the most. As an example, in Q1 LinkedIn doubled the number of hires from the year-ago quarter, and, as per Sordello, investors can expect more of the same.

Of course, there's nothing wrong with spending to fuel growth -- assuming the expected growth both comes to fruition and warrants the level of expenses incurred. Will today's dramatic jump in costs pay-off? That's what LinkedIn investors ought to keep tabs on.

Too many eggs in one basket
One of Weiner's stated objectives -- as it should be -- is to further diversify LinkedIn's revenue streams. Currently, LinkedIn's sales come from its three divisions: Talent Solutions, Marketing Solutions, and Premium Subscriptions. Progress appeared to be made in Q4 when LinkedIn reported its primary revenue-driver Talent accounted for 57% of total revenues, down from the prior quarter.

Unfortunately, the improvement in spreading the proverbial wealth took a couple of steps backwards in LinkedIn's first quarter. Now, Talent makes up 62% of total sales. The culprit? A relative drop in revenues from LinkedIn's Marketing unit. At the end of 2014, Marketing revenues were climbing as a percentage of total sales -- but that positive momentum waned last quarter, dropping to 19% of overall revenues from Q4's 22%.

Considering the acquisition, the reliance on LinkedIn's Talent division to drive future growth will likely become even more pronounced. Are questions surrounding ROI of, spiraling overhead, and an over-reliance on one business unit reasons to shun LinkedIn stock? Not necessarily, but prudent investors would be wise to monitor these key areas going forward.