From the Wikipedia link you gave:
In a report published in 2001, the European Parliament stated that the
term ECHELON is used in a number of contexts, but that the evidence
presented indicates that it was the name for a signals intelligence
collection system. The report concludes that, on the basis of
information presented, ECHELON was capable of interception and content
inspection of telephone calls, fax, e-mail and other data traffic
globally through the interception of communication bearers including
satellite transmission, public switched telephone networks (which once
carried most Internet traffic) and microwave links.
Also, NSA Watch has collated a wide range of Echelon evidence, however you'd need to decide for yourself whether or not you trust NSA Watch more than Wikipedia.
ZDNet provided an article on Echelon, but it is more speculative than evidence.
Cyber-rights.org has probably the best link to Echelon data I can find.
I'd be pretty confident that a lot of the evidence will not make it to the public eye, as it is likely to be quite sensitive around intel sources, so we may need to take the European Parliament's word that the evidence does suggest it exists (or existed)