You know the IPO drill: First-day pop, then quick fizzle. The question is, what's worth buying (or selling) once the initial hubbub is over? While investors will have to wait until spring to get in on the highly anticipated Facebook IPO, we look at a few popular companies that are already trading shares.

Buy: Spirit Airlines

The airline debuted last May at $12, but analysts say Spirit Airlines (SAVE) is poised to fly higher. Spirit is known for its affordable flights around Florida, Latin America and the Caribbean (it recently offered $11 one-way tickets on some routes), but it also charges for water and overhead-bin space. It uses those small charges to keep fares down, says Bob McAdoo, an airline analyst at Avondale Partners. The carrier recently announced an agreement with Airbus to add 75 new planes to its fleet, which analysts say will allow for expansion in its popular vacation market. Annual revenue is forecast to grow around 25 percent in 2012, to $1.3 billion.

Sell: Pandora Media

Since 2000, Pandora Media (P) has used its Music Genome Project to create Internet radio station lineups so listeners get songs they love. But the stock has been facing the music since its IPO at $16 in June. New rivals such as Spotify are giving the Oakland, Calif., firm a run for its money, with Spotify providing 15 million songs, 17 times as many as Pandora. Some analysts value the stock at $4 a share. Pandora CEO Joe Kennedy says Spotify isn't a direct competitor and that 4 percent of radio listening is done through Pandora, up from 2 percent in 2010.

Hold: LinkedIn

One of the most anticipated IPOs of 2011, the stock of social-networking website LinkedIn (LNKD) is still up more than 50 percent from its $45 debut in May. In November, the firm sold 8 million more shares at $71 each in a secondary offering. Some analysts have been skeptical of the firm's valuation; LinkedIn's price-to-expected-earnings ratio is 137 (the P/E of the S&P 500 is 12). Ken Sena, an analyst at Evercore Partners, expects long-term growth for the firm but says the stock is fairly valued right now. LinkedIn declined to comment about its stock, but last fall CEO Jeff Weiner said the firm aims to push its profit margins much higher.