This answer mentions that you can profit from investing at the right time with the right companies related to Lester's assassination missions as Franklin.
Friend Request is the first mission the player does with Lester and its description mentions how Lester will make bank as a shareholder when Jay Norris is assassinated -- it's unclear how, but it's a small detail not mentioned.
In the real world, Facebook (LifeInvader) and Twitter (Bleeter) are sort-of competitors because they are huge moguls in the same sector -- I imagine, if something drastic happened to one, the other might see an opposite reaction in its stock price.
Is this the case in GTA V too? If I buy stock in Bleeter (or a LifeInvader competitor, if there's another), will its price go up for a profit?
I plan to test this and write an answer, but I may as well ask first, in case anyone else knows.