eh, the valuation on facebook I understand though. Yes, it is extremely high, but the premise is that with that much social data, that much user stickiness, and that level of talent, and you are one of very few players that look like they could seriously give google a run for their money in the advertising business. Read enough, and you start wondering if Verizon / Google / Facebook won't outright own digital advertising industry in enough time. Read some more fanciful stuff, and you start wondering if they have enough data to actually change user behavior and dictate spending streams preemptively.
I could be wrong on all of this though, I'm just a dev, not an industry expert : )
But if you look at Facebook or Twitter, it is hard to tell why they're selling for that amount.
I myself avoid index funds which over-invest in tech because I believe we're in a bubble.