Sure you can use any other 3rd party developed blocks at popfly.com or wrap/encapsulate an WSDL or REST API in your Popfly block and use that block to create your mashup. For instance try http://www.networkmashups.com.
As you are using a 3rd party component you are bound to the availability of that component in your mashup. The risk is present, but you can minimize it by testing your mashup. Of course there are APIs, such as the Flickr API, which have almost no downtime because is part of Yahoo. You can who is the authority thta host the third party component and it is serious to you then use that component / API. In the end is a matter of trust.
