The lust from investors for getting in on the popular social-networking sector reached a fever pitch when LinkedIn (NYSE:LNKD) debuted on Thursday. Within minutes, the stock more than doubled from its initial $45 pricing, and they were recently trading around $105.

Time will tell whether social networks and the businesses that thrive because of them will stand as solid stocks in investment portfolios, but right now, the rush is on.

All eyes are, unsurprisingly, on Facebook. The company represents the Holy Grail of social network IPOs and rarely does a day go by that somebody floats their theory on when — and at what price — the company will finally make a public offering. At this point, a Facebook IPO is inevitable, and when it comes, it’ll be an event in the Internet technology business akin to when Google (NASDAQ:GOOG) went public in  2004.

However, investors shouldn’t let Facebook blind them to the rest of the social networking business pool, especially three other companies likely to go public in the next 18 months. These companies are fairly well known, and investors should keep them on their radar.

Yelp

For a time, it looked as though consumer-reviews website Yelp was going to stay a private concern, continuing to rake in capital from a variety of partners and fending off the acquisition advances of Internet titans like Yahoo (NASDAQ:YHOO) and Google. Last month, a story in The Wall Street Journal reignited investors’ hopes for a public offering though, with CEO Jeremy Stoppleman spelling it out in black and white: “An IPO is back on the table for us.” The company is currently valued at around $530 million by analysts like Next Up Research, though others place it higher.

Groupon

Given the blitz of attention on Groupon and its role as the leader in the popular daily deals business — seen as something of a threat to Yelp’s local business reviews site — it’s likely that a number of investors are already salivating for a public offering from this company. Sources quoted in an April Reuters report said that a Groupon IPO was likely for the second half of 2011 with Goldman Sachs and Morgan Stanley leading the buy up and ultimately valuing the company at between $15 and $20 million.

Zynga

Zynga is one of the companies that became monumentally successful in its own right despite its victories coming in large part from Facebook. The makers of Facebook games like Farmville, Cityville, and Mafia Wars have spread their tentacles onto mobile phones as well as web browsers. The company completed its last round of funding in February, raising $500 million and valuing the company at around $10 billion. While all signs suggest that an IPO is coming, it will likely be in early 2012 rather than later this year.

As of this writing, Anthony John Agnello did not own a position in any of the stocks named here. Follow him on Twitter at @ajohnagnello and become a fan of InvestorPlace on Facebook.