PPS comparison means nothing since it depends on outstanding shares. Your initial market cap comparison shows LNKD has 6x greater valuation...so an apples to apples share price in ZNGA dollars would be a PPS for LNKD at $15.60. Also LNKD has half the cash as ZNGA. HOWEVER... the elephant in the room is that LNKD is demonstrating SIGNIFICANT top line growth and Net growth. Their sock price is being rewarded for this... if ZNGA was demonstrating the same rate of revenue growth (or bookings) then it would likely be at this level ($15ish) or higher based on their cash.
Now... saying all that.... if ZNGA can hint at future revenue growth through a new product/service/etc. then it will climb significantly from here which is rock bottom (or was rock bottom before today).