Generally I would annualize risk and returns even when an asset's returns/general time series (ts) does not span over the full year So, both, FB and G present risk and return over the past year. For risk and return that is calculated over longer periods I would not include an asset in the portfolio of which you have no ts available to measure risk and returns. So, if you try to asset the 5 year portfolio risk and return I would not include asset FB and G in the portfolio. Already extrapolating to full years is making assumptions some may find pushing the envelope.
I would strongly advise not to go the correlated asset replacement route. Over a long-term horizon, such as your 5 year risk/return calculations, there is no replacement asset that can be correlated too highly with your asset, whose longer return time series are missing, to make up for unaccounted unsystematic/company specific risk.
A simple calculation should make this clear: Lets take portfolio of 3 assets A,B, FB. Let's assume you have the portfolio risk and return over the past 5 years but not the individual asset time series over 5 years for FB. Now, replace the missing time series FB with a highly correlated asset. Re-calculate the portfolio risk and return profiles. Derive the tracking error to the true portfolio risk and return profiles. Now, before plugging in the correlated asset returns into the missing time series spots, introduce a one-time jump of +-20% at an arbitrary location in the time series. Re-calculate your portfolio risk and return, derive your tracking error. Are you happy with the results? Can you live with the induced jump? Because if you say no you should never even start to think to replace asset returns with any correlated asset returns. 20% moves due to corporate actions, or any unsystematic even for that matter is highly conservative, generally you witness a lot higher deviations over such long period of time, plus correlations generally completely break down after such large moves, something we did not even account for in our back-of-the-envelope calculation. This all assumes we are talking about cash equity as an asset class. Other asset classes may witness higher or lower jumps, and the 20% is purely arbitrary, though as pointed out I believe it is at the low end of what can happen in a 5-year time span.