The S&P/ASX 200 (Index: ^AXJO) (ASX:XJO) has struggled to break into positive territory today and at the time of writing was trading 0.26% lower to 5,443 points.

The biggest drags on the market today have come from the energy and materials sectors and this has more than offset the good gains that have come from the financial and REIT sectors.

Four shares that have come under serious selling pressure today, include:

Vocus Communications Limited (ASX: VOC)

After a disastrous performance yesterday, shares of Vocus have fallen another 7% to $4.05. This means the company has now lost more than $1 billion worth of market capitalisation in the space of two days. The market is clearly unhappy with the company’s latest update which suggested its most recent acquisition of  NextGen isn’t performing to expectations. The shares have now lost 57% since mid-June, when they hit their 52-week high of $9.51.

Amaysim Australia Ltd (ASX: AYS)

The theme of poorly performing telecommunications companies has rubbed off on Amaysim today with its shares falling more than 8% after a weaker-than-expected market update. Although the company expects to add around 60,000 new users for the first half of FY17, it expects the average revenue per user (ARPU) to be lower than FY16. On a more positive note, Amaysim is on track to launch its broadband offering in early 2017.

Fortescue Metals Group Limited (ASX: FMG)

Shares of Fortescue have shed around 4% today after the price of iron ore fell 4.4% overnight. The bulk commodity has enjoyed a stellar run over the past few months and this has seen a massive resurgence in the share prices of Fortescue, BHP Billiton Limited (ASX: BHP) and Rio Tinto Limited (ASX: RIO). As highlighted here however, the outlook for iron ore remains unclear at the moment and there is a good chance investors could witness further falls over the coming weeks.

Sirtex Medical Limited (ASX: SRX)

Sirtex Medical shares have fallen more than 2.8% today following a broker downgrade. Morningstar has downgraded the shares from a buy to a hold following the biotechnology company’s recent research and development (R&D) investor update. Nevertheless, Sirtex expects to deliver another year of double-digit dose sales growth in FY17 and looks like a good long-term prospect for risk tolerant investors.

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Motley Fool contributor Christopher Georges owns shares of Amaysim Australia Ltd, Sirtex Medical Limited, and Vocus Communications Limited. The Motley Fool Australia has no position in any of the stocks mentioned. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.