Because of the many scams in it. This will lead to wild price fluctuations that will have an impact on investors. but, if used to like and share facebook pages and all facebook posts about ICO I think that's no problem.
Scam isn't the excuse why Facebook and Twitter are against the ICO, it's because of the goverments and banks. Actually ICOs are the most effected way of cryptos to take over the market and of course stop it will also stop the developing of cryptos in generally.