Hmm, first-mover advantage maybe, but there's no real "network effect" I can see - are all my friends on CL? How would I even know? Craigslist is heroically ugly, but I wouldn't call that "user hostile" - the only obvious dysfunction I can see are the usual gaming of listings a la eBay etc., from local car dealerships spamming keywords, etc.
Network effects in a marketplace are clear. Buyers go where the sellers are, sellers go where the buyers are, etc.
Craigslist networking is mostly local, so some places there's not much in the way of listings (for jobs, services, marketplace, etc) and some places only one category is popular and some places you would find your job, your apartment, your furniture, and before personals went away, maybe your spouse.
