My preferred way of determining if public Internet companies are cheap or expensive on a relative basis is to look at two key ratios: enterprise value / forward-year revenue and price/earnings-to-growth ratio, or PEG, (using forward-year earnings).
The ratios are used by professional investors in lieu of the more traditional price-to-earnings ratio featured on consumer financial sites. They also solve two important issues:
1) Some recent IPOs include companies that are not profitable, making P/E comparison impossible. Therefore, as a first pass, looking at revenue allows you to comp a company like Twitter against its peers. Twitter EV/FY '14 revenue is 21x, and its next closest peer, Yelp, was at 13x. Facebook and LinkedIn trade at an EV/FY '14 of roughly 10x. (Yahoo Finance has EV on its Key Statistics tab and FY revenues on Analyst Estimates.)
You need to poke around most finance sites to find enterprise value and forward-year estimates; I'm not sure why these ratios are not more front and center. Having P/E prominently located sort of reminds me of how they still teach Latin but not computer programming in grade school. Time to change the curriculum.