Student caving did OK long before the internet came along. Read the magnificent series of ULSA Reviews, for example. (There are many other examples.) But the typical student age group of today does use Facebook (etc) a lot, so if it helps this generation, then great. But will a full record of this generation's contribution to caving still be easily available to cavers in a few decades time, if it relies on Facebook?
As I suggested above, the best situation is a combination of electronic media and permanent paper publications.
There is another problem which no-one's mentioned (unless I've missed it). The immediacy and convenience of Facebook posts often leads to them being hurried and they may not be that well thought out. (There are exceptions to this generalisation, of course.) If information is submitted to a proper journal (whether electronic or paper) then good editing often occurs, ironing out poor spelling & grammar, removing ambiguity and reconciling facts with other sources (either already published or within the same draft journal). The result, although you may have to wait for it, may then be much more reliable as an information source than Facebook posts.